Adding accessible text to column headers in users -> search (Bug #1266924)

Adding accessible text to:
- the profile icon column
- the select users checkbox column

Change-Id: I663478e05588bdd3d34ab2995465d6bbf7ba3359
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>
Merge "Adding accessible text to column headers in users -> search (Bug #1266924)"