Added wiki create route and link in wiki nav

Updated the wiki_create route so that the path is allowed to be empty.

The controller for this route uses a new form WikiCreateType and data WikiCreateData that includes this path if it is specified in /wiki/_create/{path}. The template for this controller will now have the path as a text input.

If the path is empty, then it can be freely edited. It must validate to the wiki page regex, and the WikiPage entity now has this regex as a constant.

Otherwise, the path text input is disabled, and the legacy functionality remains intact for all routes with a path matching the regex.

Finally, the wiki navigation has expanded to include a link to the wiki_create route with an empty path if the user role is granted. This is the same role the controller uses.