Add noopenh264 to platform, mountpoints for openh264 extension

Updated description

Adds noopenh264 stub library to the platform in order for it to be replaced by the openh264 binary provided by an extension. This MR used to also add the extension itself, however there are concerns about versioning that are blocking merge, and this is a release blocker.

As a result I am splitting the extension itself into a separate MR until we decide how this will work. This means that release is not blocked, but our announcement about what to do with codecs will be a bit messier :P

I have retained the old description below, as gitlab does not allow edit history. It's worth noting that as we don't deprecate the ffmpeg-html5 extension here, it will need to be supported for 19.09, but in my opinion this is probably good from the perspective of a packager. Only arch users (or people who built a new flatpak from source) will benefit from the openh264 extension, and for everyone else packagers will have to include ffmpeg-full with the app. This gives some breathing room to allow distros to update flatpak, reducing churn for downstream.

Old description, incl. extension

Adds a platform extension to download the openh264 binary from cisco, which is placed in the stub from noopenh264.

I don't think it's a good idea to fully deprecate the ffmpeg-html5 extension yet as this requires flatpak 1.4.2 or newer, which is nowhere near being available in a lot of distros (e.g. debian).

Part of #815 (closed)