Add Jira Connect branches controller

What does this MR do?

This MR is part 3/3 for #2647 (closed). For a full-context MR (all of these MRs combined), see !64890 (closed).

In this MR, we add the backend, namely:

• a new controller, branches_controller.rb
• The Haml template for the jira_connect/branches/new route
• Add the JS file to initialize the Vue app
• A feature flag and specs.

UX will be reviewed in the full-context MR: !64890 (closed). Any required changes as a result of the UX review will be made in follow-up MRs.

⛳ - this MR is behind feature flag jira_connect_create_branch, disabled by default and scoped to user actor.

Manual Testing

1. Setup a Jira Cloud instance.

2. Open this branch in Gitpod.

3. Install the app manually in Jira from the Gitpod URL (/-/jira_connect/app_descriptor).

4. Open a Jira issue and check that the "Create branch" action doesn't show up.

5. Enable the FF in Gitpod and reinstall the app.

6. Open a Jira issue and see the "Create branch" action in the right sidebar:

   

7. Click it, create a branch, and once it's synced back to Jira reopen the issue and see the branch(es):

   

8. Click the dropdown menu to create another branch:

   

To do

• [-] Need to provide the Jira URL to the frontend, so that we can give the user the option to return to Jira.
• Tests, probably a feature spec to test the whole flow, and some basic unit tests for the controller. The frontend parts look like they already have solid coverage! 👍
• Some refactoring to reuse code from normal issues.
• A feature flag check.
  • Since the feature is enabled through the app descriptor I'm not sure how this will behave, but according to https://developer.atlassian.com/cloud/jira/platform/connect-app-descriptor/#app-descriptor-updates Jira will regularly poll the app descriptor and publish a new version when it changes, so I think this should work.

Iteration	MR
1. frontend Add supporting Vue components	!66034 (merged)
2. frontend Add the parent Vue app and initialization JS	!66036 (merged)
3. backend Add the backend	🚀 This MR

Screenshots or Screencasts (strongly suggested)

Happy path:

jira-current

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
• [-] I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• [-] I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Related to #2647 (closed)