Knowledge is power, man!

The config.js cache predates the feature base/known-domains.
Technically, it's also able to recall more domains that the feature
recent-list can (because the latter limits its entries).

Moves the things around to be able to override the config with the URL
params specified in the hash part of the location URI to which the app
is navigating to.

They will be stored in redux and the PageReloadOverlay will be displayed.

Note that this commit also introduces a subtle (and yet important!) change:
the location URL is now always set, regardless of the configuration loading or
not. This is needed in order for the retry logic to pick it up.

The error is stored in the redux store in base/config so other components can
consult it. It is also broadcasted as a new event in the external API for the
SDK.

This patch loads the config later than we used to, that is, only once we
know the room the user is about to join.

Due to architectural limitations in lib-jitsi-meet, it needs to be
initialized with a configuration in order to properly function. This is
unfortunate because we need to create a video track in the welcome page,
but don't know the room (hence no config) yet. In order to circumvent
this problem an empty configuration is used, which is later swapped with
the appropriate one, once loaded.

Some interesting side-effects of this change are a perceived speed
increase when the app starts or a conference is hangup. They are both
due to the fact that no config needs to be fetched from a remote server
in those cases.

The config object defined by lib-jitsi-meet is not used by
lib-jitsi-meet only, jitsi-meet respects its values as well.
Moreover, jitsi-meet defined classes and/or functions which manipulate
that config object. Consequently, it makes sense to move the config
object and the associated classes and functions in a dedicated feature.