a11y imporvements to wiki/history.html

https://edx-wiki.atlassian.net/browse/LMS-1309

1. When a revision diff details are toggled on the focus is moved to the diff details div. Previously bootstrap collapse() was being used which changed the url anchor before displaying the corresponding details div causing screen readers to start reading from the start.
2. Added scope attribute to th elements to associate header information with the cells. See http://www.w3.org/TR/html4/struct/tables.html#h-11.4.1 for details.