A11y: made "Show By" buttons for assignments list more verbose

Fixes CNVS-13138

Test plan:
- Log in as a student.
- Using a screen reader, verify that the "show by" radio buttons have labels
  that say "show by date" and "show by type", and not simply "date" or "type".

Change-Id: I615b0e10eede1cbf4d00274d1ff5c74ffb5ea3e7
Reviewed-on: https://gerrit.instructure.com/48153
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins
QA-Review: Nathan Rogowski <nathan@instructure.com>
QA-Review: Adam Stone <astone@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>

app/views/jst/assignments/ToggleShowBy.handlebars

@@ -7,16 +7,18 @@
     id="show_by_date"
     name="show_by"
     type="radio"
+    aria-label="{{#t}}Show by Date{{/t}}"
     {{checkedIf showByDate}}
   >
-  <label for="show_by_date">{{#t "date"}}Date{{/t}}</label>
+  <label aria-hidden="true" for="show_by_date">{{#t "date"}}Date{{/t}}</label>
 
   <input
     id="show_by_type"
     name="show_by"
     type="radio"
+    aria-label="{{#t}}Show by Type{{/t}}"
     {{checkedUnless showByDate}}
   >
-  <label for="show_by_type">{{#t "type"}}Type{{/t}}</label>
+  <label aria-hidden="true" for="show_by_type">{{#t "type"}}Type{{/t}}</label>
 </div>
 {{/if}}