app: add color bar on bottom of message bubbles, set standard message spacing

[redshiftzero]

Description

Sets standard message spacing and message width as speced in zeplin. Closes #466

Adds a tiny 5px color bar (as defined in zeplin) to the bottom of each message:

and the reply will turn coral on reply failure:

Closes #365 (will file followups for the other reply failure tasks)

Test plan

Test the GUI changes seem reasonable, and also verify the reply failure case updates the UI (I put in an abort(500, 'TEST') here to test)

Also follow STR from #466 in order to test that the message spacing is now uniform 28px and cannot push down the color bar.

Checklist

If these changes modify code paths involving cryptography, the opening of files in VMs, network (via the RPC service) traffic, or fine tuning of the graphical user interface, Qubes testing is required. Please check as applicable:

• I have tested these changes in Qubes
• I do not have a Qubes OS workstation (the reviewer will need to test these changes in Qubes)
• GUI changes only, no qubes testing needed

[redshiftzero]

This should be ready for review, and it also addresses #466 - though be aware of the other outstanding bug on master #467 which will impact testing (short version: restart the app if you submit a new test message on an existing source). Since #466 is addressed here this is fine to merge since it is to do how the conversation view is drawn, the other outstanding bugs are to do with when the conversation view is drawn.

[sssoleileraaa]

gorg!