AppArmor: give the Tor Browser "Web Content" process access to the system-wide...

AppArmor: give the Tor Browser "Web Content" process access to the system-wide webext directory (refs: #15717)

… otherwise uBlock cannot be loaded once we confine these processes under their
own AppArmor profile.

config/chroot_local-includes/usr/share/tails/torbrowser-AppArmor-profile.patch

@@ -227,8 +227,8 @@ index fe95fdb..7ebf9d6 100644
 +  /etc/xul-ext/** r,
 +  /usr/local/share/tor-browser-extensions/ r,
 +  /usr/local/share/tor-browser-extensions/** rk,
-+  /usr/share/xul-ext/ r,
-+  /usr/share/xul-ext/** r,
++  /usr/share/{xul-,web}ext/ r,
++  /usr/share/{xul-,web}ext/** r,
 +
 +  /usr/share/doc/tails/website/ r,
 +  /usr/share/doc/tails/website/** r,