User search column headers should include text to explain sort order

[Jono M]

In Administration -> Users -> User search, results can be sorted using any of the column headers in the table. However, these headers give no indication to screen reader users as to how the users are being sorted (which sort header is active, which order the users are being sorted in, etc).
Hidden text should be added to the active column header so that screen reader users know how the users are sorted - "sorted ascending"/"sorted descending" would work.

Affects latest master

[Mahara Bot (dev-mahara)]

Patch for "master" branch: https://reviews.mahara.org/3096