Matrix client improvements

[derivmug]

This PR adds a few things to the matrix client:

• Display leave messages
• Option to show/hide leave/join messages
• Add list of room members on pressing RIGHT in the room menu
• Add option to write a message to the bottom of each chat

[codecov[bot]]

Codecov Report

Merging #149 into devel will increase coverage by 0.04%. The diff coverage is 15.62%.

@@            Coverage Diff             @@
##            devel     #149      +/-   ##
==========================================
+ Coverage   40.57%   40.62%   +0.04%     
==========================================
  Files         243      244       +1     
  Lines       19215    19393     +178     
==========================================
+ Hits         7797     7878      +81     
- Misses      11418    11515      +97

Impacted Files	Coverage Δ
apps/matrix/main.py	17.07% <15.62%> (-1.37%)	:arrow_down:
splash.py	7.69% <0%> (-0.31%)	:arrow_down:
apps/system_apps/systemctl/systemctl.py	4% <0%> (ø)
ui/tests/test_menu.py	86.27% <0%> (+1.08%)	:arrow_up:
apps/settings/main.py	33.98% <0%> (+7.67%)	:arrow_up:

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[CRImier]

Thank you!