Entry editing tabs get info messages

Similar to the previous commit where entry editing tabs received
identifiers, this commit adds per-tab information messages.
Those messages should be translatable (not tested, though) and
can be multi-line, where the pipe symbol '|' acts as line break.

First use case is a message that directs the user from the
'Publication' tab to the 'External' tab in order to add DOIs or
URLs to an entry.

Info messages may contain links like '<a href="#...">..</a>'.
Future commits will add functionality to make use of those links.