Allow the wiki to be configured as publicly editable
Problem to solve
Allow open source projects to benefit from community contributions to their wiki without opening their repository to commits from strangers.
Non-code-contributors to open source projects.
Currently the Wiki's permission model does not align well with the needs of a typical open source project. In such contexts the wiki is often used as a common space for users and developers alike to collaborate on documentation, proposals, and the like.
However, currently only users with developer role are allowed to edit the wiki. Even worse, non-developers are barred from viewing page history (a somewhat orthogonal issue tracked in gitlab-ce#14620).
Granting end-users the developer role simply to allow them to edit the wiki is an extremely large hammer and one which opens the project to a variety of security threats from unintended code being merged to the tree.
Introduce a per-project configuration option to allow the wiki to be publicly editable. When enabled this option will allow guests to create, edit, and delete pages in the project's wiki.