Invites a11y fixes

[sstrigler]

Fixes prompted by an accessibility audit funded by NLnet (done by HAN University
of Applied Sciences).

[sstrigler]

HAN Accessibility Report - JWChat.pdf

[coveralls]

coverage: 18.497% (-16.0%) from 34.463%
when pulling 32aaab3 on sstrigler:invites_a11y_fixes
into 5426358 on processone:master.

[coveralls]

coverage: 18.497% (-16.0%) from 34.463%
when pulling 32aaab3 on sstrigler:invites_a11y_fixes
into 5426358 on processone:master.

[coveralls]

coverage: 34.457% (-0.004%) from 34.461%
when pulling 75c66cb on sstrigler:invites_a11y_fixes
into 531d17d on processone:master.

[sstrigler]

Things are never perfect. Areas to improve are feedback from registration form. Especially for invalid username and weak password. But I think this should be handled at a central place and give users a better idea of what went wrong and how to fix it. Didn't want to put too much effort in this because it would also mean a lot of struggle with translations. So better to do it once and right.