Update session history when switching branches in pipeline editor

What does this MR do?

For #322485 (closed)

When the user switches branches in the pipeline editor, the branch_name param in the URL is updated and pushed to the session history. This allows the user to retain the same branch they are working on when they refresh the page.

Note: If the user initially loads the page and they are in the default branch, there is no branch_name param in the URL.

Branch Switcher

The breakdown of the implementation for the whole branch switcher feature is as follows:

Link	Status	Progress
!57562 (merged)	✔	Add dropdown UI to the pipeline editor
!57941 (merged)	✔	When clicking on a branch, switch to that branch and load the correct data
!60465 (merged)	✔	Add branch switcher to empty state
!59217 (merged)	WIP	Fetch and search available branches in pipeline editor
	you are here 👉	Retain the same branch on refresh

Local Testing

The feature is under the :pipeline_editor_branch_switcher feature flag, so please enable it first when testing.

• Go to CI/CD > Editor to view the pipeline editor.
• Switch branches around and confirm that the URL changes.
• Branch should be the same when reloading the page.

Note that the branch switcher is currently using a client resolver. Make sure to test with a branch included in the resolver, or add your own test branch in pipeline_editor/graphql/resolvers.js.

Screenshots

Screen_Recording_2021-05-05_at_17.12.49

Does this MR meet the acceptance criteria?

Conformity

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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
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Security

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