polish message bubble styling

[redshiftzero]

Couple of remaining styling tasks for the message bubbles:

• apply padding as speced in zeplin within the message bubble
• apply box-shadow as speced in zeplin: note the box-shadow CSS option is not supported by Qt so this will need some research

Font changes and the user badge are tracked separately

[eloquence]

@ninavizz Could you link to the preferred Zeplin that shows the intended final message bubble styling?

[eloquence]

Stale issue as padding was tweaked e.g. in #496, #1029, #1059. Closing, we can track any further tweaks in new issues.