OAuth2 authentication work so far

This PR is still WIP (see tasks below), but it does allow a user to successfully authenticate against the joind.in website via OAuth.  This code also relies on a [website PR](https://github.com/joindin/joind.in/pull/756) being merged to identify failed authentication (so that the app can capture that and inform the user accordingly).

The new code uses the built-in Android account management system to store credentials.

To get this code working, there is an example file in `source/res/raw/oauth.dist` that should be copied to `source/res/values/oauth.xml`.  Create an API key on the joind.in site, and use `joindin://oauth-response/` as the callback URL.  Edit your `oauth.xml` file and pop your API key into here, then build the app.  This should allow authentication and redirection back to the app.

Things left to do on this PR:
- [x] Tidy up code - there's some IDE auto-comments and similar
- [x] Remove any redundant strings in `strings.xml` that were from the previous username/password preferences
- [x] Adjust the 'denied' capture code based on the website PR mentioned earlier, if any changes are required
- [x] Document the `oauth.xml` file usage somewhere, so it's more obvious what is required to do for build.