make expanding explanation accessible

LMS-11161
Toggle aria-expanded attribute
Fix inconsistent ui issues on discussion during blackout period
FOR-108
Added link from instructor dashboard to analytics dashboard
Annotation Tools: Adding Loading Bar to Annotations Table

- added label below loading gif

 - fixed spacing issues

  - change the error messages in annotator code
Added enrollment breakdown by certificate track
Separated August changes into new table in chg log
List of all email messages and options to view and copy them
Corrected duplication and sp in change log

@mhoeber can I get a thumbs?
Merge branch 'ahodges/documenttion/chglogfixup' of github.com:edx/edx-platform into ahodges/documenttion/chglogfixup