Replaced PluginCheckError with ServiceCheckFailed
Removed artifact event lines
Addded migrations.
Updated field help texts
Fixed faulty fields on forms
changed default check intervals
Added new interval_stays_ok for setting a custom interval when service state stays OK.
Also improved logic flow for determining if a notification needs to be sent + added documenntation.
Added help text for notification interval settings.
fixed interval and last_sent determination logic
changed old STATE_UP to STATE_OK
Fixed build_messge method for notification plugins
Fixed super call for ServiceException
Fixed event determination and trigger logic + improved logging.
Removed debug ipdb statements.