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.

calendar/templates/month_mini.mustache

@@ -51,8 +51,9 @@
                     }} title="{{#str}}monthprev, calendar{{/str}}"{{!
                     }} data-year="{{previousperiod.year}}"{{!
                     }} data-month="{{previousperiod.mon}}"{{!
+                    }} aria-label="{{#str}}monthprevwithname, calendar, {{previousperiodname}}{{/str}}" {{!
                 }}>
-                    <span class="arrow">{{{larrow}}}</span>
+                    <span class="arrow" aria-hidden="true">{{{larrow}}}</span>
                 </a>
                 <span class="hide"> | </span>
                 <span class="current">
@@ -65,8 +66,9 @@
                     }} title="{{#str}}monthnext, calendar{{/str}}"{{!
                     }} data-year="{{nextperiod.year}}"{{!
                     }} data-month="{{nextperiod.mon}}"{{!
+                    }} aria-label="{{#str}}monthnextwithname, calendar, {{nextperiodname}}{{/str}}" {{!
                 }}>
-                    <span class="arrow">{{{rarrow}}}</span>
+                    <span class="arrow" aria-hidden="true">{{{rarrow}}}</span>
                 </a>
             {{/includenavigation}}
             {{^includenavigation}}

lang/en/calendar.php

@@ -181,7 +181,9 @@ $string['mon'] = 'Mon';
 $string['monday'] = 'Monday';
 $string['monthly'] = 'Monthly';
 $string['monthnext'] = 'Next month';
+$string['monthnextwithname'] = 'Next month: {$a}';
 $string['monthprev'] = 'Previous month';
+$string['monthprevwithname'] = 'Previous month: {$a}';
 $string['monththis'] = 'This month';
 $string['more'] = 'More';
 $string['moreevents'] = '{$a} more';