Implement the MessageOfTheDay functionality completely in the plugin

The MessageOfTheDay extension point was removed from Gerrit core. Thus,
this plugin was not working anymore with Gerrit version 3.0 or higher.

This change moves the complete handling of the messages to the plugin.
The messages are still configured in the
$GERRIT_SITE/etc/messageoftheday.config-file and are given in html
files in the $GERRIT_SITE/data/messageoftheday directory. The latest
configured message will be provided by a REST API endpoint implemented
by the plugin. A UI element using the 'banner' endpoint will show the
message as provided by the REST API.

Change-Id: I05f7e7c672389f809ee6ea143656d5335dd8ffe0
10 files changed