Polls: finishing touches (prop mutation, proper state after voting, UI color adjustment)

Closes #1021 (closed)

What does this MR do?

• UI: Recolor blue info alerts to dark color scheme
• Vue: use data prop pattern and a callback to show correct state right after voting
• UX: display info text to explain why own votes are not visible afterwards

How confident are you it won't break things if deployed?

very.

Links to related issues

see everything else in &34 (closed) 😄

How to test

see #1021 (closed)

Screenshots

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• set a "for:" label to indicate who will be affected by this change
• use "state:" labels to track this MR's state until it was beta tested
• added an entry to CHANGELOG.md
• part of the voting-tool release notes
• see vote betatesting group