Get rid of Apache Commons

[rfc2822]

We're nailed to old versions of Apache Commons as long as we don't increase the minimum SDK. These libraries have various problems which are already fixed in new versions, but we can't use them.

Also, they are not optimized for Android and/or Kotlin.

So maybe it's possible to replace them by (in order of preference):

1. Kotlin methods
2. Java methods
3. Android / support library methods
4. Guava methods (guess we will still need a general-purpose support library; however we can always use the latest Guava version without problems as I understand it and it's optimized for Android)

[rfc2822]

#883 has removed the explicit dependencies, but the Commons are still used (imported over dependencies of dependencies).

• Scan through all code directories for "commons" imports and replace them