fix(external_api): Fix screensharing for firefox 74

[Arzar]

Starting from version 74, firefox need to be explicitly allowed to capture the screen from an iframe for screensharing. This new Feature-Policy is called "display-capture".
See: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Feature-Policy/display-capture

[jitsi-jenkins]

Hi, thanks for your contribution!
If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.

[saghul]

@jallamsetty1 can you please check if this doesn't break Chrome or older Firefox releases?

[jallamsetty1]

@jallamsetty1 can you please check if this doesn't break Chrome or older Firefox releases?

Sure, will do.

[jallamsetty1]

Works on Firefox 73, no errors/warnings seen there.
Chrome throws a warning that it's an unrecognized feature. However, screensharing works fine w/o any issues.

[saghul]

I’d say let’s land it then, since it’s in the W3C doc: https://www.w3.org/TR/screen-capture/

[joekerna]

For me even with this PR there is still no screen sharing button in Firefox 74.
It's working fine in Firefox 73 so I guess the configuration is correct?!