Track merge request changes UI usage with Amplitude

Follow up of https://gitlab.com/gitlab-org/gitlab-ce/issues/46030, to have actual events to be tracked.

Description

• Track/send events to Amplitude:
  • When a merge request's changes (diff) tab/view is loaded, send an event with event type projects:merge_requests:show/VIEW_CHANGES. This can be caused by any way you can view the changes tab/view. It can be loading it directly (deep-linking to the tab), or if a user clicks on the tab.
  • When a merge request's changes (diff) tab is selected (via click / keyboard navigation), send an event with type projects:merge_requests:show/SELECT_CHANGES_TAB.
  • When a merge request's changes dropdown file selector is opened (via click / keyboard navigation), send an event with type projects:merge_requests:show/OPEN_CHANGES_FILE_SELECTOR.
  • When one of the files in that selector is selected (via click / keyboard navigation), send an event with type projects:merge_requests:show/SELECT_CHANGES_FILE_SELECTOR_FILE.
  • When a user focuses the search field (via click / keyboard navigation), send an event with type projects:merge_requests:show/FOCUS_CHANGES_SEARCH_FIELD