Migrate merge conflicts modal

What does this MR do?

A tiny MVC for #229677 (closed) where we migrate all the old HAML code for this modal to a new Vue app. This effort is broken into 5 merges.

What	Merge
Add new modal and replace mr_widget_header	!47889 (merged)
Update mr_widget_conflicts
Update mr_widget_conflicts_warning	TBA
Update mr_widget_conflicts_help	TBA
Remove _how_to_merge.html.haml and extra JS	TBA

Screenshots (strongly suggested)

Before	After

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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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

Related to #229677 (closed)