Also extracts static emoticon cache to a field in order to increases
testability of filter_emoticon, because we are now able to reset the
static emoticon cache before tests, particularly for observing the
response to changing $CFG->emoticons.
- Complete the deprecation information to follow the rules.
- Minor tweak to PLAIN tests to have clearer expectations.
- Completely delete get_global_config() from filters, there
was not much utility keeping it there. Note we have opted
for direct deletion because it's private API, not public one
(and searching github/google/docs did not reveal any use
of those functions either, just in case).
