Add intermediate level of wiki permissions (BB-8404)

Currently wiki permissions have three levels:

• None i.e. no wiki enabled
• Private i.e. only those with repository access can view and edit
• Public i.e. anyone can view and edit

It would be useful to add a new level between Private and Public which would have public view permissions but private edit permissions, what exactly such a level would be called I am not sure. Maybe Public Viewable and rename Public to Public Editable?

This would be particularly useful when the wiki is used as the documentation hub and you want anyone to be able to view the documentation but only a limited group of people to be able to edit the wiki.

Just today I spotted a user randomly edit a wiki page and then revert their edit which was a little disconcerting because I hadn't realized just how permissive the Public setting is, but since I am using the wiki for documentation I want it to be at least public viewable.