Add an option for polls to allow users to vote for multiple items

[dsevillamartin]

Fixes #3

Changes proposed in this pull request:

• Add option for allowing multiple option votes from users
• Add option for max votes allowed per user if the poll allows multiple option votes
• Reorganize help section display to always have icons and show multiple instances, plus max votes
• Creates new API endpoint due to the inability to adapt the existing one without breaking changes
  • Same with Flarum and Pusher events

Reviewers should focus on:

• The performance of the new endpoint is probably not great in larger scale, due to the constant refreshing of vote counts. This could be changed to increment & decrement (preferably moved to a queue as well, though then users would see the wrong count and polls with a few votes would look wrong). At least, only the options that were changed (added/removed) are refreshed.

Screenshot

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).

[dsevillamartin]

New changes make the poll look like this. I know a 'max votes' option could probably be standalone without a switch for multiple votes, but that switch might be better UX? Unsure. I don't think it's a big concern regardless.

[dsevillamartin]

I've improved the performance of the new endpoint.
These metrics are with the poll screenshoted above - two options. The # of selects goes up like 1 per additional option after this improvement, before it went up by 2 per option.

# OF QUERIES	Voting	Unvoting	Changing Vote
Before PR (legacy endpoint)	21	19	24
Before optimization	21 (~14 ms)	19 (~12 ms)	24 (~15 ms )
After optimization	19 (~8 ms)	17 (~6 ms)	21 (~9 ms)

This performance seems to be better than the legacy endpoint.