• Refactor using CaptchaClient as an example
• Add client test using CaptchaClientTest as an example

Note: doesn't depend on apps-android-java-mwapi (and therefore doesn't block dropping the dependency) but still need to be moved to Retrofit.

This task isn't quite "Easy" but it's a good candidate for volunteer contributors looking to get more familiar with the codebase, MediaWiki API, and unit tests