0️⃣ Make ChannelConnection.is_active nullable so it can be eventually removed

[rowanseymour]

Was about to remove ChannelConnection.created_by and noticed we also never use this - connections like sessions are hard deleted. So we can 1) make it nullable 2) stop setting it in mailroom 3) remove it

[nicpottier]

man the scourge of smart models sure has deep tendrils

[codecov]

Codecov Report

Merging #2286 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #2286   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files         221     221           
  Lines       26344   26344           
======================================
  Hits        26344   26344

Impacted Files	Coverage Δ
temba/ivr/tasks.py	100% <ø> (ø)	⬆️
temba/channels/models.py	100% <100%> (ø)	⬆️

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