David Mudrák authored Jan 11, 2019 and Sara Arjona committed Jan 21, 2019

The useridfrom would normally be a foreign key to the users table. But
it can also contain some extra negative values with special semantics
(-10 or -20 for no-reply or support user). Therefore we can't make it a
foreign key and the index must be created explicitly.

Toni Barberà Melià authored Jan 11, 2019 and Marina Glancy committed Jan 18, 2019

This commit is part of work on Custom fields API,
to minimize commit history in moodle core the work of a team of developers was split
into several commits with different authors but the authorship of individual
lines of code may be different from the commit author.

The capabilities changed ('contextlevel' => CONTEXT_COURSE changed to
'contextlevel' => CONTEXT_MODULE) are:

* moodle/site:accessallgroups
* moodle/site:viewfullnames
* moodle/site:trustcontent
* moodle/site:viewuseridentity

This list came from reviewing the _get_extra_capabilities functions in
all core activities. They were all somewhat inconsistent, but I think it
makes sense that these capabilities are consistently overridable in all
activities. E.g. moodle/site:accessallgroups affects conditional
availability even if there is no other user of groups, and
moodle/site:viewuseridentity and moodle/site:viewfullnames affect the
logs report, if nothing else.

As a result of this, several _get_extra_capabilities functions are no
longer needed, and all the rest have been simplified.

Amended to keep it on-sync with 3.6.1 emergency release

This is to replace usages of external::data_for_messagearea_contacts
in the new UI.