Better accessibility for User search filter by initials (Bug #1266920)

- Added the aria-labelledby attribute (read by orca)
- Added the aria-label attribute (not read by orca but seeing if it
will be read by Jaws)
- Added an accessible hidden span to alert screenreader what is to
follow

Change-Id: I21e48b9cce3998e3435b157e144a6ece9c47974b
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>
Merge "Better accessibility for User search filter by initials (Bug #1266920)"