refactor hide / delete actions

[der-john]

Strongly related to #2374 (follow-up: introduce DELETE), and thus to #2336 (physical deletion).

The original aim of this was:

• translate the popup saying 'Do you really want to delete this?' when you hit 'delete' on an IdeaCollection proposal.

While doing this, I noticed that hide/delete functions were wrapped in extremely similar ways (including the aforementioned question) in the following five places:

• IdeaCollection
• Alexanderplatz
• Blog
• Comment
• Mercator2015

@xi and I discussed the possibility to factor out this functionality. I suggested adding a 'resourceAction' - eventually this action should replace adhDeleteAction, since HIDE should be used where 'mark-as-deleted' used to be used.

In this PR,

• a resourceAction called adhHideAction is implemented;

the following use cases are replaced with adhHideAction:

• IdeaCollection
• Alexanderplatz
• Blog (physical deletion #2365 replaced 'mark-as-deleted' with physical deletion. The
  use case for this was deleting badge assignments, not blog contents.
  Therefore @2e2a and I suggest to replace the delete action in Blog.ts with hide, even though I couldn't test it right now.)

The following use cases are left untouched:

• Comment - because the template of the hide action is different (it's an icon).
• Mercator2015 - because the functionality is part of a ResourceWidget.

TODO:

• check for remaining use cases of 'mark-as-deleted' (-> none),
• check for remaining use cases of DELETE (-> none),
• check for remaining use cases of adhDeleteAction (-> only Mercator2015).
• check whether Mercator2015 can be changed affordably.

[der-john]

rebased.