MDL-46416 group: Use a revision number for group pictures
MDL-48189 grade: letters now covered by behat
MDL-48191 gradebook: fixed floating headers when emails not displayed
MDL-48276 qformat_xhtml: avoid uncessary html in .css file

In order to help automated tools move the html out of the css
file.
MDL-48277 css: initial csslint config file
MDL-47329 mod_book: use embedded pagetype instead of hardcoded HTML
MDL-47329 mod_book: missing RTL styles, added page body selectors
MDL-48210 plugin_manager: Invalidate caches after fetching updates info
MDL-48210 updates checker: Improve cron notifications processing

These are cosmetic improvements spotted while working on the issue.

1. Do not waste time if there are no changes to notify about.
2. Fix the legacy plugin manager's get_plugins() call.
MDL-22309 accesslib: Adding unit test to expose the issue
MDL-22309 accesslib: Preventing incorrect get_role_users() uses
MDL-22309 report_singleview: Fixing wrong uses of get_role_users()
MDL-38128 mod_assign: Adding group selector to grading summary page

Thanks to Pau Ferrer Ocaña and Avi Levy for
their work on this issue.
MDL-43127 tool_uploadcourse: Add support for more enrolment methods
MDL-41155 Lesson: Added behat tests for multiple question attempts
MDL-41155 Lesson: Fixed multiple question attempts logic

Thanks to Rajneel Totaram for providing this patch.
Merge branch '48189-28' of git://github.com/samhemelryk/moodle into MOODLE_28_STABLE
Merge branch 'MDL-38128_28' of https://github.com/dmonllao/moodle into MOODLE_28_STABLE
Merge branch 'MDL-43127-28' of git://github.com/FMCorz/moodle into MOODLE_28_STABLE
Merge branch 'MDL-46416-28' of git://github.com/FMCorz/moodle into MOODLE_28_STABLE