Add system/alerts endpoints

[jamespfennell]

• List all alerts in a system
• Get an alert by ID

Will need to

• Honor the alerts_detail URL parameter (probably should not allow NONE!)
• Add a boolean URL parameter alerts_active_only default to true.
• Support this being false in the relevant alert query
• Add a new alert view or reuse an existing one with all_active_periods
• Add links to the existing alerts.

[jamespfennell]

Will need to introduce an alert uid for this to work.

[jamespfennell]

This is done in the Go migration!