Cron management UI

[Alan Noble]

VidGrind requires a UI for creating, editing, and deleting crons.

NetReceiver performs this at the site level under its settings page. Although VidGrind provides a way for users to select sites, it currently lacks any site-level UI.

Most crons manipulate variables which are in turn associated with devices. It is therefore proposed that crons be initially managed on the Device page.

If a need is subsequently found for site-wide crons, the functionality can be moved accordingly.

Note: This issue relates to cron management only, not the functionality to execute crons.

[Alan Noble]

Upon further consideration, a separate Crons page under settings is preferable since there is no straightforward way to limit crons to just devices.

[Alan Noble]

Resolved in https://bitbucket.org/ausocean/vidgrind/commits/00a794e37f6326d71fe795896abc0289b3e6fcff