For a long time, Moodle has had the feature to force the language
for a whole course. This change adds the same feature at activity
level.
The course-level feature was controlled by a capability
moodle/course:setforcedlanguage, and I decided to use the same
capability to control this feature. I think a new capability would be
overkill.
If the allcountrycodes filter contains only invalid values, ignore the
whole filter setting and make get_list_of_countries() return the full
list of all known countries, rather than empty list.
Includes:
* no more hacky reloads, everything is written only once and kept until cache reset
* lang menu list is now cached in MUC
* both string and lang menu caches are compatible with local caches on cluster nodes
* config-dist.php cleanup
