Base support for syncing to alternative trackers

[DakuTree]

With MAL slowing going down the tubes, I think it's time to look at supporting other sites.

The main two sites I'm looking to support off the bat are Kitsu (#341) and AniList (#355), as they have been highly requested and also the only decent alternatives.

At the moment, it isn't just a simple case of adding another service. A lot of the code for syncing assumes MAL is used, and other trackers actually have a working API we can use for updating, rather than using our current method of CORS. So it all works a bit differently.
Current idea is just to set it up like we have sites. Problem is just making it all work.

Other issues:

• Where do we store credentials (API Keys)? Server or Browser
  Both have issues for different reasons. Server means we can technically send update requests from the server, but do we even want to do that. It also allows avoids the need to setup credentials on each PC.
• Does the server or browser make the sync request?
  At the moment it's handled by the browser as we aren't using an API, however with an API the server could handle that, although it doesn't seem like the best idea as it may get us IP banned.

Other misc notes:

• Kitsu Docs: https://kitsu.docs.apiary.io
• AniList Docs: http://anilist-api.readthedocs.io/en/latest/lists.html

Looking to start work on this soon after I get settled in at my new job.

[doutatsu]

Any updates on this?

[Josexv1]

Update please.

[doutatsu]

@DakuTree forgot about us 😭