Better slug unique warning for Documents

As editors may be editing documents with the same title we needed to
improve the functionality of this warning. This commit changes the
warning text to an alert to be more visible, disables the Publish button
if it is not unique and only makes the uniqueness check happen if the
document isn't currently Published. This means that you can still
Publish already published documents.
Better slug unique warning for Manuals

Same process as Specialist Documents. Disable the publish button if
there is a slug clash, move the warning to a banner and change the
slug check to only be displayed on the draft Manual.
Merge pull request #344 from alphagov/better-slug-unique

Better slug unique warning