[CIS-1038] Fix composer allowing whitespace only messages to be sent

[b-onc]

Composer send button should be disabled when the message is only whitespace. Sending whitespace only message breaks our UI.

Also, backend trims the whitespace when the message is published. If we send the message with leading/trailing whitespace, the UI first shows the message, but when backend sends the actual message, UI jumps. So it's better to send trimmed message to backend, to prevent UI jumps.

[codecov-commenter]

Codecov Report

Merging #1293 (13fc30b) into main (11ed592) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1293   +/-   ##
=======================================
  Coverage   91.30%   91.30%           
=======================================
  Files         219      219           
  Lines        9492     9492           
=======================================
  Hits         8667     8667           
  Misses        825      825           

Flag	Coverage Δ
llc-tests	91.30% <ø> (ø)
llc-tests-ios12	91.30% <ø> (ø)

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[nuno-vieira]

LGTM!