Tabs > HAML > Migrate app/views/projects/merge_requests/creations/_new_submit.html.haml & app/views/projects/merge_requests/_page.html.haml to GlTabs

Merge requests render tabs in two different contexts:

• When creating a new MR (app/views/projects/merge_requests/creations/_new_submit.html.haml).
• When editing an existing MR (app/views/projects/merge_requests/_page.html.haml).

Those views implement tabs in slightly different – both non-compliant – ways.

As part of this issue, both views should be migrated to use Pajamas-compliant tabs. This is a non-trivial effort which doesn't have a clearly defined migration path.

The current logic relies on some Javascript bindings in app/assets/javascripts/merge_request_tabs.js.

A possible way to go about this would be to consolidate both views to render the tabs' markup with the gl_tabs_nav, gl_tab_link_to and gl_tab_counter_badge helpers, and refactor app/assets/javascripts/merge_request_tabs.js to make use of GlTabsBehavior.