Minor UI improvements / fixes for epic creation

What does this MR do?

Fixes two minor bugs reported in #37028 (closed) and #37005 (closed):

• Placeholder for adding epics to an epic incorrectly shows #epic id instead of &epic id
• "Title" text box in "New epic" dropdown does not auto-focus (This was probably caused as Vue and Bootstrap dropdown don't play together very nicely, fixed by using the event from bootstrap)

Screenshots

new-epic-focus

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] 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

Closes #37028 (closed), #37005 (closed)