Project Wiki complete deletion

Description

As disabling the Wiki permission on Project Settings will not remove the Git repository of Wiki physically, this issue suggests complete deletion of the Wiki.

Initial state		Disabling the Wiki Permission
	=>

Proposal

Even by removing the project we cannot remove the Git repository of the Wiki at this moment (as of GitLab 10.3) according to gitlab-ce#14256.

UPDATED: At the moment (GitLab 10.3), even when the Wiki feature is disabled on the Project Settings, the Wiki repository will not be cleaned up but leave it alone so that the project owners can revive the Wiki without data loss by re-enabling the Wiki permission.

It would be nice to have "Remove the Wiki repository" is available on the Project Settings for the project owners apart from disabling the Wiki permission.