webui : improve accessibility for visually impaired people

[ngxson]

Fix #13531

NOTE: we don't aim to be WCAG compliant because we're currently lack of time & efforts to do that. However, what we can do is to make the UI usable with a screen reader

I used MacOS VoiceOver to test this, still not very happy about it because I don't know if it works well on all browsers.

At least on Firefox, I can select every button now (using tab) and their labels are being read correctly.

On safari, it just tells that there is a button without a label, which is quite annoying. Also, safari skip entirely the "upload file" button next to the "send message" button

Also, maybe because I don't know how to use a screen reader, but toast messages don't seem to be picked up (even though they use correct aria-live ; the toast notification function is provided by react-hot-toast package) - A simple test case is to click on the "send" button without entering any message, it should read an error "Please enter a message"

@domasofan can you check if this is now usable?

[isaac-mcfadyen]

toast messages don't seem to be picked up (even though they use correct aria-live ; the toast notification function is provided by react-hot-toast package)

Likely related Issue on the react-hot-toast GitHub: timolins/react-hot-toast#335