Transcoding at lower bitrate for limited bandwidth (#58)

Hello!

To begin with, thanks a lot for Funkwhale and the Android application, which is delightful to use every day.

As mentioned in #58, the application does not yet have a way to reduce the amount of data exchanged between the player and the server. When streaming high-quality audio on 4G or 5G connection, it can get very costly if one does consume a lot of FLAC content. This PR is an attempt at solving this issue and is my first contribution to the project. I would appreciate some hints/helps according to unit tests, translations, and probably things I did not cover for the moment...

Here is a summary of what I have done:

• Add a new setting called "Bandwidth limitation" with an icon, and a basic description,
• If bandwidth limitation is set to limited, append &to=mp3&max_bitrate=320 to the listen URL of the candidate best track upload.

A better way to do it in the future would be to add a bar to configure the desired maximum bitrate, and probably check in the cache if the best upload is already there before asking for a reduced one. Testing that the best quality upload is already meeting the bandwidth constraint would be interesting to avoid an unneeded transcoding on the server side. I prefer to point out the flaws so that we can discuss about them together! 😄

Thank in advance,

Dylan