Introduce DESKTOP_SHARING_FRAMERATE_AUTO to allow browser to determine sharing fps automatically

[cedricroijakkers]

This PR Fixes #1668

The docker containers allow the option to set the screen sharing fps via two environment variables DESKTOP_SHARING_FRAMERATE_MIN and DESKTOP_SHARING_FRAMERATE_MAX. This is fine for people who want to tweak these settings, but on meet.jit.si, these options are left to the defaults. These are that min and max framerate are not configured at all, and that they are therefore left to the browser to determine.

This PR introduces a new environment variable DESKTOP_SHARING_FRAMERATE_AUTO which when set to true, which is the default, does not configure the desktop sharing parameters in the web container at all, i.e. leaving them up to the browser to determine (which is also the behaviour on meet.jit.si). For those who still want to configure their own framerate, they can set DESKTOP_SHARING_FRAMERATE_AUTO to false and configure DESKTOP_SHARING_FRAMERATE_MIN and DESKTOP_SHARING_FRAMERATE_MAX as they wish.

[saghul]

Thank you!