Activity extension for favorites

[PVince81]

Trigger activities on favorite/unfavorite.
Add column in the activity page “send change notification only for favorited files/folders”

Extend OCP\Activity\IExtension for that.

CC @nickvergessen

[nickvergessen]

I could take care of this if you want me to

[PVince81]

That would be nice, thanks.
But first we need to get the basic work merged: #2

[PVince81]

The main part of the favorites features has been merged in core: owncloud/core#12360

[nickvergessen]

Btw, does this really make much sense?
I mean we can easily do this, but in most cases you know that just marked it as favorite.

De-favoriting may make sense, so you can find the file quickly again and then re-favorite it, but that's what undo actions should be for.

[nickvergessen]

We don't really want that, self actions are disabled by default anyway.