Introduce and use a helper for the full path to a wiki

The following discussion from gitlab-ee!2807 should be addressed:

• @nick.thomas omas started a discussion:

  Hmm, I can't find a helper method for this offhand. project.wiki.path doesn't use the full_path.

  I see this pattern is repeated quite a few times in the source code, so let's have a separate issue to introduce a helper method and refactor everywhere constructing this path.

  We don't need to let that delay this MR though.

Currently we repeat this manual path construction in a number of places, which is bad. project.disk_path + ".wiki" isn't hard to add - perhaps as project.wiki.disk_path?