Improve pipeline triggers UI
What does this MR do?
This updates Pipeline Triggers to behave as described in: https://gitlab.com/gitlab-org/gitlab-ce/issues/27156.
The API, backend is done.
Sub-tasks
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update "add trigger" and "edit trigger" forms and workflow - 
add placeholder text to input (as shown in design) 
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shouldn't need an "are you sure" confirmation to edit 
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should show the first several characters of the token on the edit page 
 
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update "revoke trigger" and "take ownership" workflows - add a more descriptive confirmation dialog than "are you sure?"
 
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update trigger token list - 
should fit cleanly in the space available (maybe use flexbox?) 
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description should overflow with ellipsis and tooltip instead of wrapping 
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edit and revoke buttons should be made into icons 
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token should be truncated to show only 6 characters 
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should display "legacy" next to legacy tokens 
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should display "invalid" next to tokens for which the user doesn't exist 
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should display "invalid" next to tokens where user has insufficient permissions 
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"invalid" and "legacy" tags should should have appropriate tooltips 
 
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[ ] update build page sidebar- tokens should only show 6 characters
 
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add test for "add trigger" workflow - 
should create a new token with current user as owner 
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description should match, also should not be allowed to be empty 
 
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add test for "edit trigger" workflow - 
should be able to view edit trigger page with trigger description 
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should update description and save trigger 
 
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add test for "take ownership" workflow - 
should work on legacy tokens 
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should work on non-legacy tokens 
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should work on "invalid" tokens 
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should not be available to non-master users 
 
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add test for "show tokens" page - 
should only show "take ownership" option for tokens which are not owned by you 
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should only show "edit" for non-legacy tokens 
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should only show tokens you own when non-master 
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should only show first few characters of the token 
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should display "legacy" next to legacy tokens 
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should display "invalid" next to tokens for which the user doesn't exist 
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should display "invalid" next to tokens where user has insufficient permissions 
 
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TODO:
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Unit tests, 
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Updated frontend, 
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Updated specs, 
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Documentation 
Screenshots (if relevant)
Does this MR meet the acceptance criteria?
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Changelog entry added 
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Documentation created/updated 
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API support added 
- Tests
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Added for this feature/bug 
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All builds are passing 
 
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Conform by the merge request performance guides 
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Conform by the style guides 
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Branch has no merge conflicts with master(if it does - rebase it please)
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Squashed related commits together