Update and migrations

[binsky08]

Changes:

• update dependencies
• stability improvements
• code cleanup

In detail:

• update gradle from 4.1.0 to 7.0.0
• update ndk from 21.3.6528147 to 21.4.7075529
• replace deprecated jcenter() with mavenCentral() repository
• update other dependencies (espresso-core, commons-codec, android-async-http, junit, ion)
• fix linting issues
• don't trigger failure callback twice after failed login
• use startActivityForResult() when opening login activity, special thanks to @fynngodau
• move activities to an own package
• move adapters to an own package
• move fragments to an own package
• unify the fragment and activity class names

[fynngodau]

Thanks @binsky08 for implementing my suggestion with moving activities to an own package so thoroughly :+1: it makes it much easier to find one's way through the code :)