Adds base/sounds feature which allows other features to register a sound
source under specified id. A new SoundsCollection component will then
render corresponding HTMLAudioElement for each such sound. Once "setRef"
callback is called by the HTMLAudioElement, this element will be added
to the Redux store. When that happens sound can be played through the
new 'playSound' action which will call play() method on the stored
HTMLAudioElement instance.

It's not supposed to go on top of everything, like an error would. It's tied to
a conference, so render it outside of the OverlayContainer.

These modification are not necessarily directly related to the
containing PR, I merely saw them while reviewing the containing PR.

Do the selection in mapStateToProps so the container itself doesn't need to
receive all the props that each overlay needs.

Each overlay is responsible for fetching their own props and for providing a
"needsDisplay" static method wich will be called with the full redux state and
should return true if the overlay needs displaying.

Also eliminate duplicated state keeping: the connection and conference error
states can be fetched from their respective base features.

Recently/as part of the work on taking into account the user defined by
JWT, the local participant (on mobile) started telling the remote
participants that he/she had the display name "me". Obviously, that's
incorrect. Do not store the default display name in redux. While it may
be argues that redux is the place for all states, base/participants and
the name property of Participant is not meant to be that display name
because that is being sent to remote participants, the default name
needs to be internationalized, etc. So it's better to not store the
default display name at this time at all because it's not used by mobile
anyway and Web already deals with remote participants who don't share
their display names.

Will override email, display name and avatar URL with the values
provided in 'context.user' structure of the JWT token.

Settings will no longer be used to retrieve local display name,
email and avatar URL. Now those values will be obtained from
the /features/base/participants Redux state.

fix(jwt/middleware): use const for default name

fix: wrong default display name on web

ref(base/participants): remove getDisplayName functions

ref(jwt): do not accept unknown user fields