Post load error handler

Adds a placeholder which allows to write a plugin for executing some
code after the "load error handler" is triggered. A function named
"postLoadErrorHandler" should be defined in one of
the "#include virtual" files.

ref(avatar): Move Avatar and ParticipantView to base/participants

Split language detectors to be web/native dependent. Take care of strings that contain html.

Adds translate function with default namespaces and options.

config: switch default disableRtx to false

The error raised by JitsiMeetJS.init() is already in the state of
features/base/lib-jitsi-meet so it's not a good design to store the same
error in the state of features/unsupported-browser.

config: Don't require an extension for firefox >=52.

Starting with firefox 52, no extension is required for screensharing.
See https://wiki.mozilla.org/Screensharing

cleanup: remove old adaptive simulcast config option

`host` contains the hostname:port portion, whereas `hostname` is just the
hostname, not including the port.