Accessibility improvements

[JohnXLivingston]

I made an accessibility audit on my project. Part of the issues concern Converse.
Here are the required fixes. I have separated them in different commits, so we can discuss if necessary.

Here is the list of changes:

• adding role=button attribute on dropdown actions, for better
  accessibility with screen readers.
• Autocomplete fields: shift+tab should always give focus to previous
  focusable element, and tab should give the focus to next focusable
  element if the input is empty.
• dropdown buttons: adding a "Menu" aria-label, and adding aria-hidden on icon.
• message actions: opacity:1 on focus.
• list-item-action: opacity:1 on focus, tabindex=0 to be focusable.
• adding aria-hidden="true" on fields with hidden-username class.
• Adding role="heading", aria-level="2" on userinfo, MUC title and
  chatbox titles, to improve navigation with screen readers.
• Adding role="heading", aria-level="3" on controlbox-heading elements.
• converse-avatar:
  • Fix alt_text parameter that was ignored
  • using aria-label instead of alt attribute on svg (alt is not valid)
  • if no alt_text, adding role="presentation" so that screen readers
    ignores the image
  • when fallbacking on initials, adding a aria-label with the full name

As my project only uses Converse in singleton mode to display one MUC, the audit is incomplete.
I tried to fix some of the issues more widely than what was required for my project, but i can have missed some points.

• Add a changelog entry for your change in CHANGES.md
• When adding a configuration variable, please make sure to
  document it in docs/source/configuration.rst
• Please add a test for your change. Tests can be run in the commandline
  with make check or you can run them in the browser by running make serve
  and then opening http://localhost:8000/tests.html.