Make new OPAC alert feature available from Admin Pages

[Terran McCanna]

Wishlist: The new Bootstrap OPAC adds a alert message functionality that can be updated from the config.tt2 file. It would be even nicer if the alert could be set from within the client's admin pages when a system administrator is not immediately available. The new settings in the config file are:

# Set a maintenance message to display in the catalogue
ctx.maintenance_message = "<b>This OPAC will be unavailable sometime for planned upgrades.</b>";
# Maintenance types follow Bootstrap 4 alert options as follows
# success = green; info = blue; warning = yellow; danger = red
ctx.maintenance_type = l(danger);

[Lynn Floyd (lfloyd)]

This not only effects Bootstrap but also TPAC also. IF this is available in the client then it would be more useful for those of us that run multiple app servers.