Clearly show what options user voted for & make text more readable

[dsevillamartin]

Refs #15

Changes proposed in this pull request:

• If voting checkbox input is not shown, show checkmark next to percent
• Reorganize DOM and use grid layout for general poll option
• Remove PollOption as an absolute bar, use PollBar::after pseudoelement
• Use mix-blend-mode for attempting to get readable text color
• Fix selector highlighting voted options

Reviewers should focus on:
It's hard to find settings that work well for all scenarios - probably impossible. I believe this is an improvement over the current state, though. Some scenarios have worse legibility, but this allows those that barely had any to have some.

Maybe a setting can be introduced to let forums decide whether they want to use mix-blend-mode? Since the effects mostly depend on the selected primary & secondary colors, it might be a good idea to let them turn it off if it works better without.

Screenshot

Confirmed

• Frontend changes: tested on a local Flarum installation.