Author: Sara Arjona
MDL-61601 cohort: Add theme support for cohorts
If enabled $CFG->allowcohortthemes, then themes can be set at the cohort level.
This will affect all users with only one cohort or more than one but with the same theme.
The default theme order will be: course, category, session, user, cohort, site.
http://git.moodle.org/gw?p=moodle.git;a=commit;h=88cb8b781d83e6aff3ef275a719023dcd2ea8499
http://github.com/moodle/moodle/commit/88cb8b781d83e6aff3ef275a719023dcd2ea8499
+ 3.5 en [allowcohortthemes,core_admin]
+ 3.5 en [configallowcohortthemes,core_admin]
+ 3.5 en [invalidtheme,core_cohort]