* improved handling of dirty contexts in general - caching, switching in cron, marking, loading, etc.
* role_assing() and role_unassign() now marks dirty contexts because we use has_capability() in this function - we can not do it later outside to speedup bulk operations
* fixed some inline docs
* fixed notice from rs closing
* removed cached $CONTEXT from has_capability() - $context is now required parameter; this was hiding serious bugs when context did not exist or ppl passed false as parameter
* removed some ===, we can not use these on function parameters - we must support ints, strings, '', nulls, etc. - this could be a source of really dangerous bugs in future
* some other improvements and fixes - documented inline
* renamed Preferences to "My report preferences"
* renamed Site defaults to "Report defaults"
* rename Change site defaults to "Change report defaults"
* reintroduced Inherit option to Grader preferences - Nicolas was right, it is not possible to implement preference overrides without it with our current get_pref inplementation
* new separate option in plugin selector "Course settings"
* moved displaytype, decimals and aggregation position to "Course settings"
* created new table grade_settings + related functions grade_get/set_preference()
* user report now uses grade_seq class instead of grade_item::fetch_all(); added preloading of grade items into grade_grade instances
* other minor bugfixing/cleanup/improvements
Please note that the Grader report preferences and its defaults must be resaved again - sorry.
MDL-11527 fixed warning from $CFG->defaultuserroleid when installing and upgrading
+ removed reusing of previous contexts in has_capability() and get_context_instance() because it was hiding critical errors when context was false, not specified, etc.
+ tweaked roles defaults - faster and defaults are applied when roles exist
+ SYSCONTEXTID may be specified in config.php to eliminate 1 db query
+ static context cache is now reset after rebuilding of context paths
+ path field in context table is now nullable - pg compatibility requirement
+ other minor fixes
MDL-11566 only site shortname, fullname and desc are required/displayed during installation
MDL-11567 fixed upgradesettings redirects if error in settings
MDL-11143 defaultfrontpageroleid moved to Frontpage settings page (easier to find), speedup
* in_null for decimals defaults checks
* store letter settings only in database - no cfg settings
* separate configuration page for letters only - pluginselector+admin tree
* new capability to manage grade letters - similar to scales and outcomes
* added grade_letters db index
* grade letters related minor bugfixing
