Add release notes for 1.7.28

This covers all fixes and improvements since 1.7.27.

Testing done: Built html docs and looked at them.

Reviewed at https://reviews.reviewboard.org/r/6251/
Release Review Board 1.7.28.
Fixed the syntax for a command in the 1.7.28 release notes.

The command was showing as a broken link, since in ReST, surrounding
text with single backticks makes it a link. We now use :command:`...`
instead.