Create GitLab branch from Jira issue: UI Polish

What does this MR do?

⛳ this is behind feature flag jira_connect_create_branch, disabled by default.

This MR is the 4th part of #2647 (closed). In response to the UX review of !64890 (closed), a few changes have been made:

• When the initialBranchName is available, we add it to the title e.g. "Create branch for Jira issue ABC-123"? !64890 (comment 632112064)
• Use GlEmptyState for the success state (when branch is successfully created): !66032 (comment 632724962)

To achieve this, some minor re-organization of the frontend code has been done. We now have a pages directory which contains index.vue, and this renders our new branch form, and the success state.

To test

1. Enable jira_connect_create_branch FF
2. Navigate to http://gdk.test:3000/-/jira_connect/branches/new
3. Try to create a branch
4. Observe success state

Screenshots or Screencasts (strongly suggested)

Current state (video demo slightly outdated, the illustration has changed. See success state screenshot below):

new_branch_ui_polish

Success state

With no default branch name in URL query parameters

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.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team