Fix API project lookups when querying with a namespace with dots

Attempting to use the /projects/:id API by specifying :id in
"namespace/project" format would always result in a 404 if the namespace
contained a dot.

The reason? From http://guides.rubyonrails.org/routing.html#specifying-constraints:

"By default the :id parameter doesn't accept dots - this is because the dot is
used as a separator for formatted routes. If you need to use a dot within an
:id add a constraint which overrides this - for example id: /[^\/]+/ allows
anything except a slash."

Closes https://github.com/gitlabhq/gitlabhq/issues/9573