Add Kitsu tracker

[xgi]

Acknowledgements

• [X] Check that there is not an existing ticket for this request, and that it is not related to an existing ticket.
• [X] Create one ticket per feature. Feel free to create multiple tickets.
• [X] If this is an extension-related request, open a ticket in the extensions repo instead.
• [X] Write a concise title for this ticket in the field above.

Feature details

Add Kitsu as a tracker alongside AniList and MAL.

• Auth with OAuth2 -- should be similar to the AniList implementation.
• API docs: https://kitsu.docs.apiary.io

Additional information (optional)

No response