Fixes merge request widget date tooltip inconsistencies

Phil Hughes requested to merge mr-widget-merged-date-tooltip into master

What does this MR do?

Previously the merged request widget merged & closed states where using the merge requests updated_at date which is incorrect. This date is updated whenever the merge request is updated through comments, description changes etc.

Now the merged & closed states use their own event date which includes their own updated_at dates. These dates are also correctly formatted through our gl.utils date utility methods to create a user friendly tooltip text.

Screenshots (if relevant)

Before After
Screen_Shot_2017-09-29_at_09.47.09 Screen_Shot_2017-09-29_at_09.58.12

What are the relevant issue numbers?

Closes #38545 (closed)

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