MDL-40343 forum: indent less when mobile, nested

Changes the indenting of forum replies to be relative to the width of
the parent container, so the indent decreases as threads get nested
deeper. Also reduces the indent size automatically for smaller screen
sizes.
MDL-46219 assignment: delete files when resetting assign activities
MDL-45636 course_rest: changed modcontext to coursecontext
MDL-46088 Impossible to click on Atto in a Quiz in 'Secure' mode.
MDL-46088 Quiz: Prevent alert when clicking in editor in secure popup
MDL-46088 quiz secure window: further Atto JS fixes.
MDL-47466 calendar: use get_in_or_equal() in function calendar_get_events to fix problem with ORA-01795
MDL-46531 editor_atto: Color plugins should use classes rather than hardcoded colours
MDL-47183: viewhiddenactivities should use the module context

Thanks to Paulo Matos for the forum part of this change.
MDL-46712: Center EditPDF window on browser resize / zoom
MDL-47569 backup: Escape section summary for XML

This was causing the parsing of the XML
to fail because things like & were not
escaped.
MDL-47569 backup: Do not close questions tag if never opened

This can happen when restoring quizzes that
do not have any questions.
MDL-33939 gradingform_guide: use correct max grade for scales.
MDL-47466 calendar: fixed error ORA-01795 in calendar_get_events
MDL-30358 forms: strict validation is too strict
MDL-36987 auth_manual: fix config for field locking

Thanks to Sergey for the patch
MDL-29873 Lesson: Essays not shown on grade page with SQL server
MDL-47608 cachestore_mongo: fixed unit test error

If you configured your site to run mongo unit tests and had
the mongo extension and backend installed but did not have the
mongo service running you would get a fatal error when running
the unit tests.
That problem has been fixed by checking the mongo test instance
is ready before returning it.
MDL-46518 navigation: site glossaries now expand properly
MDL-47760 grades: dim category name if hidden