Build out a URI Adapter to allow uploading an avatar via a url

Currently only really accessible via the API. The UriAdapter creates a
tempfile from a url and gives a ActionDispatch::HTTP::UploadedFile back
to the controller to process as normal.
This will help a lot in being able to transfer avatar urls from another
app without monkey patching a lot of discourse code.
Merge pull request #1512 from ScotterC/avatar-from-url

Build out a URI Adapter to allow uploading an avatar via a url