Show small explanation next to lists of tags

Lists of tags are shown on recent changes, watchlists, history pages
and diffs. However, it's hard to tell what they are, as they have no
visual indicators and resemble edit summaries (they looks examply the
same sans the italics). Wikimedia wikis have been using various
methods to remedy that (a "Tag:" prefix in every tag message, yellow
background color, etc.).

This basically implements the first one: a link to Special:Tags and
"Tag(s):" text is shown before the list of tags everywhere it's shown.
The text is correctly pluralized depending on the number of tags given
revision has.

Change-Id: If21eaed4171e732daaee32087c3307d70b60d81f
Merge "Show small explanation next to lists of tags"