Notification of string changes

Author: Jun Pataleta
MDL-72511 calendar: Fix month_mini navigation
* Hide arrow icons from assistive technologies.
* Add a more sensible aria-label for the previous/next
month links.


http://git.moodle.org/gw?p=moodle.git;a=commit;h=aef8d9e6f14c21be83b6a3a202526a3f954310c4
http://github.com/moodle/moodle/commit/aef8d9e6f14c21be83b6a3a202526a3f954310c4

+ 4.5 en [monthnextwithname,core_calendar]
+ 4.5 en [monthprevwithname,core_calendar]