Add link to history, add edit post link, ...

* link to history added behind "timestamp"
* add edit post icon
* only display hide/delete/censor in flagging flyout
* also display restore button in flagging flyout, for now
* hide flag icon if no actions are available to the user
* refactored PostActionMenu, it was impractical to build multiple different
  buttons per actions (still not entirely happy with the new class, but it's an
  improvement)
* removed "timestamp" border-bottom, should probably only show underline, on
  hover, if there's a link

Change-Id: Ifff327500780998c0faea12f5a6f2fbb0daa4b61
Merge "Add link to history, add edit post link, ..."