new wb.utilities.ui.buildPrettyEntityLabel and cleaned up message dependencies

This introduces a new utility function wb.utilities.ui.buildPrettyEntityLabel which provides a nice
link to a Entity. By having this we get rid of some redundant code in wikibaseItem.js.
Also cleaned up some weird message dependencies and instead of using hard coded spaces, we now use
the 'word-separator' message.

Change-Id: I95a530f25f6c393ce2058993815feb0ec4d582cd
Merge "new wb.utilities.ui.buildPrettyEntityLabel and cleaned up message dependencies"