Welcome screen: optional alert set in config file

Add an optional alert to the welcome screen.

The alert is only displayed if non-empty and is configured in the
configuration file as aaf.av.welcome_alert
Mapping: revert AEPST to auEduPersonSharedToken

Revert mapping of AEPST in the sample config file
application_config.groovy.orig - partly revert
7dea5230a542fef7dd235ae63488372f9aa0ab35 - default Tuakiri ShibSP config
actually maps AEPST as auEduPersonSharedToken, I shouldn't have been
renaming AEPST to "shared-token" - that was just one peculiar config
file on the DEV host.

Correct now:
    auEduPersonSharedToken = "auEduPersonSharedToken"
Version: internal Tuakiri release 1.0-tuakiri2