Remove color and font formatting

lovetox "we could also get rid of color and font formatting"
...
lovetox "i think completely, they just dont make a lot of sense"
lovetox "you cannot know what fonts other clients support"
lovetox "nor what color themes they have applied to your system" 

https://conference.gajim.org:5281/muc_log/gajim/2019-04-03

Closes #10490 (closed).