Add Mangamo

[TheUnlocked]

Checklist:

• [x] Updated extVersionCode value in build.gradle for individual extensions
• [ ] Updated overrideVersionCode or baseVersionCode as needed for all multisrc extensions
• [ ] Referenced all related issues in the PR body (e.g. "Closes #xyz")
• [ ] Added the isNsfw = true flag in build.gradle when appropriate
• [x] Have not changed source names
• [ ] Have explicitly kept the id if a source's name or language were changed
• [x] Have tested the modifications by compiling and running the extension through Android Studio
• [x] Have removed web_hi_res_512.png when adding a new extension

This one is quite a bit more complicated than most since there's no web client to scrape so everything has to make direct API calls. I've done quite a bit of testing and it seems to work well, ignoring a couple minor cases where limitations on the Firestore API can cause issues with pagination.

[TheUnlocked]

Addressed all of the issues so far. While I was at it I also optimized queries by only requesting the fields that are used, and found a race condition in the authorization code and fixed that.