Add option to persist banner messages across server restarts

[nickdos]

When tomcat is restarted, any banner message that is set, is reverted to default state and thus lost.

• Add an option (checkbox) to persist banners across restarts

[ansell]

When I looked into the code in the past it looked like persistence was already the default:

ala-admin-plugin/grails-app/services/au/org/ala/admin/SystemMessageService.groovy

Line 15 in a87d87a

getSystemMessageFile()?.write(new JsonBuilder([message: message?.text ? message : [:]]).toPrettyString())

There may be an issue with the cache reloading at startup, see issue #5 for an open issue that may be relevant to that.