The »Activity« tab replaces the existing activity tab and merges the activities, versions and comments into a single chronological stream.

It works as follows:

• activity app registers timeline plugin which provides a timeline collection/accessor (must return ungrouped activities)
• comments module registers timepline plugin which provides a timeline collection/accessor
• versions app registers timeline plugin
• the timeline tab queries all the timeline plugins and merges the results together based on their "date" field

One idea would be to provide a base Collection implementation that can be extended by plugins to point to the correct endpoint and also transform the results, if needed.

I'll take care of writing the timeline related stuff.

(supersedes older ticket #18164)

@nickvergessen @blizzz @DeepDiver1975 @jancborchardt