Better slug unique warning

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.

This PR also adds the same functionality for Manuals.

[Ticket](https://trello.com/c/WcKBh0GO/451-frontend-slug-warning-message-in-specialist-publisher-1).