MDL-48174 core_lang: support for ->skiplangupgrade in lang cron task

Similar to upgrade_language_pack(), update_langpacks_task should
respect skiplangupgrade config. Also, this should be added to behat
whitelist, so it can be disabled during on-sync period, when lang
pack for master is not available.
MDL-30968 mod_forum: No picture link to group when no groups mode
Merge branch 'wip-mdl-48174' of https://github.com/rajeshtaneja/moodle
MDL-48174 tool_langimport: info about skipped update

So admins can work out why the task might not be having an effect.
weekly release 2.9dev