Added a JWT verification view

[Jwpe]

This PR adds a verification view: verify_jwt_token, which checks if a JWT POSTed to it is valid and that the user exists.

Much of the functionality is the same as the existing refresh view and serializer. I abstracted out the majority of the shared serializer logic to maximise DRYness. There are several other ways this could be implemented, however.

I added tests for the new view and moved a test which was more relevant to this functionality from the refresh view tests.

Suggestions for improvements welcome! If the serializer implementation is ok, it could probably use some unit tests.

[Jwpe]

Also, a lot of the logic in the base serializer is the same as in the JSONWebTokenAuthentication class - apart from the errors raised. I wonder if it would be possible/desirable to abstract those out into shared functions.

[jpadilla]

@Jwpe thanks, checking this out. I think someone tried abstracting that logic from the JSONWebTokenAuthentication a while back. I wouldn't mind seeing it done. There might also be some logic in JSONWebTokenAuthentication that can be shared as well.

[jpadilla]

@Jwpe just to be clear, were you gonna tackle the base API view in this PR?

[Jwpe]

@jpadilla happy to do so, but I can also make a separate PR if you'd rather split it up.

[jpadilla]

@Jwpe seems right to me if it were part of this one.

[Jwpe]

@jpadilla refactored to add the base class. Rebased on the latest master - I can squash the commits if you like but otherwise this should be good to go.

[Jwpe]

@jpadilla fixed!

[jpadilla]

@Jwpe this looks perfect, thanks again for the great work! Only thing missing is adding a bit on it to the docs since we are adding a new feature here.

[Jwpe]

@jpadilla added a section to the documentation. Let me know what you think!

[jpadilla]

@Jwpe looks great, thanks again!

[Jwpe]

@jpadilla no worries! Thanks for making an awesome library.