MDL-44095 frontend: fix typos in readme file
MDL-44480 Javascript: Correct dialogue ID generation
MDL-43966 events: link to migration wiki in lib/upgrade.txt
MDL-44337 glossary: behat test for glossary categories
MDL-42870 completion: show failed activities as incompleted
MDL-42870 completion: performance and readability code refactoring
MDL-30085 core_grade: added a grade web service
MDL-30085 Fixing coding style
MDL-30085 Missing and invalid comments fixed
MDL-30085 Rename core_grade component references to core_grades

Also, change since tags from 2.6 to 2.7 and fixed invalid comments
MDL-30085 Added missing MOODLE_INTERNAL and MUST_EXISTS check
MDL-44497 quiz responses report : error when no attempts
MDL-33653 questions, a way to save changes and continue editing
MDL-33653 fix preview links on calc qtype edit forms.

Also, refactor how we render preview icons, to eliminate duplication.
MDL-40913 core_message: replaced 'add contact' add_to_log call with an event
MDL-40913 core_message: replaced 'remove contact' add_to_log call with an event
MDL-40913 core_message: replaced 'block contact' add_to_log call with an event
MDL-40913 core_message: replaced 'unblock contact' add_to_log call with an event
MDL-40913 core_message: replaced 'write' add_to_log call with an event
MDL-40913 core_message: created 'message_read' event