Improve accessibility

[ZettaScript]

Currently, mumble-web is completely unusable with a screen reader. This commit improves semantic, accessibility for screen readers and keyboard navigation. (see WAI ARIA)

Further work is needed for:

• activating buttons and rooms by keyboard
• rooms accessibility
• ARIA roles

I tried several things to add a keydown event (following the MDN) in the same way click is defined, but it doesn't work.

[zDEFz]

Please add aria required true for mandatory fields like Server address. But keep in mind if it isnt visually described, you may need to address this in another pull for a GUI change to make a red star or so.

[ZettaScript]

Is aria-required needed when it is already required? As I understand it, the only difference between the two is that required prevents submitting the form and adds a "required" popup, while aria-required doesn't.

[zDEFz]

aria-required is working across screenreaders, and even older ones. My knowledge might be outdated by now, cause it appears that most screenreaders don't require aria-required instead of required for attribution anymore to read it out.

[ZettaScript]

The official recommendations say to prefer semantic HTML, and to consider ARIA only if needed. In my experience, screen reader users don't have problems with required. I think the usecase of aria-required is very rare...