Audio-only calls, video messages, editing messages, web drag-n-drop

Server

Main features:

• Unvalidated accounts can be automatically deleted now.
• keygen and tinode-db simplified.
• Support for audio-only calls.
• Inform client of required validators.

Bug and security fixes:

• Use crypto/rand instead of math/rand for generating email & phone validation codes.
• Fix for a data race in topic subscriptions.
• Several RethinkDB fixes.
• 0a6a103 Fix for #826 - login after reconnect in chatbot.

Full list of changes: v0.20.4...v0.21.0

TinodeWeb

Audio-only calls, video messages, editing messages, drag-n-drop:
https://github.com/tinode/webapp/releases/tag/v0.21.0

Tindroid

Audio-only calls, video messages, editing messages:
https://github.com/tinode/tindroid/releases/tag/v0.21.0

Tinodios

Audio-only calls, video messages, editing messages:
https://github.com/tinode/ios/releases/tag/v1.11.0