FE Simplify admin instance licenses page

What does this MR do?

CE port: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26439

Update admin license page. Now using the grape API instead of GraphQL.

Shows all licenses, with an ability to delete and download individual licenses.

Uses a feature flag.

BE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9733

Docs MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9770

Technical details

I have used composition w/ slots to achieve this in my first iteration. The table is composed of a set of cell components that have unique implementations (info tooltip, datetime, header) but are structured as table cells using the Cell component.

The Cell component is what structures the horizontal split of the cell header and body, it adds generic classes that most cell implementors will use in order to fit into the table row dynamically. This allows the rows to be flexible but provides consistency for each cell implementation. It also importantly declares slots for the composition of the unique properties of each cell type.

Screencasts

Screen_Recording_2019-03-06_at_05.01.40

Screen_Recording_2019-03-06_at_04.58.37

Screen_Recording_2019-03-06_at_05.02.35

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/7054

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer