[IMPROVE] Voice messages improvements

[gerzonc]

Proposed changes

Change voice message container to m4a which gives correct length in Android, IOS, Chrome, Edge and Firefox
Enable background mode for voice playback, so listening to voice messages is possible when the app is put into background

Issue(s)

Closes #2586
Closes #2578

How to test or reproduce

• Record a voice message with the app at IOS or Android. The message length is correctly displayed on IOS, Android and the web client
• Playback a voice message and put the app into background (i.e. by changing to another app) - the playback continues

Screenshots

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves a current function)
• New feature (non-breaking change which adds functionality)
• Documentation update (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING doc
• I have signed the CLA
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works (if applicable)
• I have added necessary documentation (if applicable)
• Any dependent changes have been merged and published in downstream modules

Further comments