fix(accessibility): poll and checkbox structure

[joshuai96]

When using a screen reader the polls are not easily perceivable.

This is related to WCAG Success Criterion 1.3.1 Info and Relationships.

• Checkbox changes:
  The checkbox component now uses the given id, name or a random id to link it's input to the label. This further optimizes the perception with assistive technologies.
• Poll:
  The question and answers were not grouped together logically enough for assistive technologies like a screen reader. Wrapping in a form and fieldset with a legend binds them together better instead of the ul element. The poll result is logically still an unordered list.
• PollsList:
  The message There are no polls in the meeting yet. Start a poll here! was wrapped in a <span>. According to the WCAG compliance test, a <span> must only be used inside of <p> (paragraph) to mark specific text passages. A single <span> element as simple text wrapper is discouraged.

This is the revive of #15645

[jitsi-jenkins]

Hi, thanks for your contribution!
If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.

[joshuai96]

@saghul
As discussed I'll mark you on the PRs that might be re-opened.

This is a revive of the other issue, lots has changed regarding accessibility, so i re-did the changes for clean PR.