MDL-39754 copy 2.5 requirements to Moodle 2.6

Optional features and warnings may be added later,
but we need at least required environment for 2.5.1

This includes database family cleanup and more realistic memory requirement.
Merge branch 'w27_MDL-39754_m24_evn26' of https://github.com/skodak/moodle into MOODLE_24_STABLE
MDL-39546 Lesson module: fixed matching question issues.
MDL-39546 Lesson module: removed duplicate to decode special characters
Merge branch 'get_users_by_capability_fix_m24' of http://github.com/timgus/moodle into MOODLE_24_STABLE
Merge branch 'w27_MDL-40266_m24_arrayshift' of https://github.com/skodak/moodle into MOODLE_24_STABLE
MDL-37818 - RSS - Prevent access to blocks while RSS context errors are displayed
MDL-39628 Chat: check mod/chat:chat capability when using daemon.

Thanks to Francois Gauthier for report/fix.
Merge branch 'w27_MDL-40399_m24_msarrayshift' of https://github.com/skodak/moodle into MOODLE_24_STABLE
MDL-39570 mod_feedback - Missing privilege check
Merge branch 'MDL-40297_24' of git://github.com/stronk7/moodle into MOODLE_24_STABLE
Merge branch 'wip-MDL-40353-m24' of git://github.com/marinaglancy/moodle into MOODLE_24_STABLE
MDL-40434 calculatedmulti multiple-response questions not randomised
MDL-40439 Merge English strings from the en_fix language pack
Merge branch 'MDL-40439-en_fix_24' of git://github.com/mudrd8mz/moodle into MOODLE_24_STABLE
MDL-37516 Conditional activities: user field condition display fixes
MDL-37516 Translate fieldnames on output only.

This reverts a change performed by the previous commit where user
fieldnames were being translated (and cached) when fetched,
causing the information to remain static later, not responding
to language switches (and perhaps leading to other problems if those
structures are used lated for DB / $USER comparisons.

With the commit, the translation happens on output, so no
potential interferences will happen and, also, the information
changes dynamically on lang switching.
MDL-40275 core Improve documentation for block_base::has_config()
MDL-38314 repository: Unit Tests for delete_all_for_context()
MDL-38314 repository: Delete repository instances on context deletion