MDL-43639: Prevent user updated event on every login

If auth plugin prevents local passwords, then user
is updated and event is triggered on every login.
MDL-45952 feedback: non-respondents in groupings

Fixes feedback non-respondents list to show only members in groups in
the selected grouping if "Available for group members only" is selected.
MDL-46633 chat: Chat send button gets stuck

Conflicts:
	mod/chat/gui_ajax/module.js
MDL-46979 backup: remove duplicate string.

Thanks to Matt Sammarco for spotting this.
MDL-39840 environment: add support for environment.xml in plugins

New features:

* environment.xml files in all types of plugins
* support for plugin strings via new optional plugin attribute in FEEDBACK
* support for class_name::method_name custom callbacks and classloader
Merge branch 'MDL-43639_updatePassEveryLogin_26' of https://github.com/moodlerooms/moodle into MOODLE_26_STABLE
Merge branch 'MDL-46979_26' of git://github.com/timhunt/moodle into MOODLE_26_STABLE
Merge branch 'wip_MDL-46679_m26_addonenv' of https://github.com/skodak/moodle into MOODLE_26_STABLE
Merge branch 'MDL-46633_m26' of https://github.com/shashirepo/moodle into MOODLE_26_STABLE
Merge branch 'MDL-45952_26' of git://github.com/Syxton/moodle into MOODLE_26_STABLE
MDL-46619 Forum: only output last poster's name if user can view it.
MDL-46619 Forum: link to the discussion only if users cannot see posts.