fixes #3978

trying to play 'none' ringtone resulted in the default ring tone being played

fixes #3972 fixes #3801 fixes #3931

the 'strings' resource on transifex was in the internal 'Android 1' format
instead of the more modern 'Android 2' format.

This according to transifex support caused some weird issues…

The only work around (apparently) was to create a new resource (now call
main-strings) and use that instead.

I hope we didn’t mess anything up in the process.

Let's be extra careful with the next release

Conversations used to deduplicate disco queries based on their hash.
However that relies on the first query to go through (device to actually
respond) and to respond properly (hash matches).

Creating a proper retry behaviour for this is actually quite challanging.
(which one would you try next, how long do you wait?)

now that we have minSdk=21 we can bump retrofit and okhttp

Closes #3988

it doesn’t seem to be possible to make the splash screen use the configured
theme but we can follow the system’s theme on supported Android versions