Count contributions prior to user joining event

[psrpinto]

Fix #161

Summary of changes

• Move stats-related code to a Stats namespace
• Add is_active() and is_past() methods to Event
• Add a Stats_Importer for importing stats
• Add Stats_Importer::import_for_user_and_event( $user_id, $event_id )
• When a user attends an event that is active at that moment, call import_for_user_and_event()
• Make sure it's not possible to attend/un-attend past events

Testing instructions

1. Login as User A
2. Create a translation
3. Login as User B
4. Create an event and make it start now
5. Login as User A
6. Navigate to the page of the event you created above, and click the "Attend" button
7. Create a translation
8. You should now be listed as a contributor, and the stats table should show a single translation (it should not count the first translation, created above in 2., as the event hadn't started yet)

[akirk]

Works well! I took the liberty to add fb3ddfc as it verifies that translations added before the event are not added retroactively.

[akirk]

Apologies, you already had a translation before the event started but it was made ineffective by my other suggestion to just create the event in the past. Fixed it now.

[psrpinto]

Thanks for removing the sleep()! I couldn't figure out how to manipulate the date_added of the translation, so that's why I resorted to using the sleep(). Much better now.

[amieiro]

In the translations table (GlotPress plugin), if you have a row with user_id != user_id_last_modified, this means this string has been reviewed by a validator, so we should add this work to the event_actions table, only if the validator is assisting to the event.

Issue has been fixed