Allow events filter to be set in the URL in addition to cookie

What does this MR do?

Supersedes https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5699, and https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13342.

• 0c28f513 2018-09-06 | Refactor EventFilter and increase its test coverage [Rémy Coutable]
• 9a813c1c 2017-08-07 | Allow events filter to be set in the URL in addition to cookie [Igor Kapkov]

Notes:

• The source of truth is still the cookie value, as we now always set the cookie in ApplicationController.
• EventFilter now ensures that the filter used has a valid value. For instance, before if you'd set the event_filter cookie to foo, the events list would be empty. Now it will defaults to the all filter in this case.

Questions:

• Do we need to document the available params?: event_filter=all|push|merged|issue|comments|team

Screenshots

Dashboard activity filtering	Group activity filtering	Project activity filtering

What are the relevant issue numbers?

Closes #1801 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides