irc: Implement the UTF8ONLY IRCv3 specification

[progval]

https://ircv3.net/specs/extensions/utf8-only

[codecov-commenter]

Codecov Report

Merging #1688 (2f6d827) into master (009a288) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1688   +/-   ##
=======================================
  Coverage   36.01%   36.02%           
=======================================
  Files         209      209           
  Lines       83973    83975    +2     
=======================================
+ Hits        30246    30251    +5     
+ Misses      53727    53724    -3     

Impacted Files	Coverage Δ
src/plugins/irc/irc-server.c	38.70% <100.00%> (+0.29%)	:arrow_up:
src/gui/gui-line.c	51.05% <0.00%> (-0.45%)	:arrow_down:

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[flashcode]

Thanks, merged!