- updated forum portfolio code to use files api and added forum export caps.

MDL-16002 rewritten upgrade locking and better upgrade progress tracking; MDL-16070 Do not use $a[0] syntax in lang packs + various other upgrade improvements and fixes

MDL-10107   Fixed the definition of message providers ... it now works through files very similar to those for events and capabilities, which makes them easy to add/remove/change as necessary

MDL-13936 indicate that user needs group membership to add new discussion, but only if having the capability to add new discussion - previously the message was incorrectly shown even when not having the cap in separate mode, visible mode needs this message too after previous commit; new lang string added in case user selects all participants in group selector and does not have accessallgroups - the existing group membership string could not be reused; merged from MOODLE_19_STABLE

MDL-13880 Overridden and excluded grades are now coloured with alternate row colours that override the default alternate row colours (gray). I also fixed the long-standing notice about aggregationposition. See attached patch.

Added message string for when a user does not have permissions to view forum discussions in a forum.