bug#38831: IceCat: some codecs don't work without workaround

["Jakub Kądziołka" <kuba@kadziolka.net>]

Hello,

I had some problems with video codecs in IceCat 68.3.0-guix0-preview1.
For example, consider this page: http://demo.nimius.net/video_test/. By
default, the videos under the headings H.264 / AAC and MPEG4 don't work
("No video with supported format and MIME type found.").

The following steps make the first of these videos work:
1. Open about:config
2. Click "I accept the risk!"
3. Set security.sandbox.content.read_path_whitelist to /gnu/store/
   (the trailing / is important).

The instructions were originally sketched out in this help-guix
message:
https://lists.gnu.org/archive/html/help-guix/2019-12/msg00150.html

I believe it would be beneficial to make this a default.

On IRC, bandali suggested that it would be better to only whitelist the
necessary store subdirectories. I don't know how to gather such a list,
but it it seems like a good idea.

I don't know how about:config entries modified by the user behave when
IceCat is updated, but in some of the behaviors I can imagine, the
config entry stops updating, in which case it would be better to add
the paths to some internal whitelist (I reckon such a whitelist already
exists and contains something like /usr/lib).

Regards,
Jakub Kądziołka

CC: mhw as suggested by nckx