Add support for jr:preload

[lognaturel]

Preloads to support

• uid: Add uid preload for instanceID #278
• datetime:
  • start
  • end
  • date
• property:
  • deviceid
  • username
  • phonenumber
  • email

Notes

There's a lot of conceptual overlap with events and actions, see #303. Can likely be treated as an alternate representation of setvalue triggered by an odk-instance-first-load event.

property preloads read values from client state. JavaRosa has a flexible way for clients to define different properties. Strictly speaking, we only need to support the ones listed above that Collect does. There have previously been requests for defining custom metadata.

For deviceid, we probably want Web Forms to take ownership of creating and storing the value in local storage. The goal is to uniquely identify a browser install so it should be shared across all Web Forms from the same domain.

username, phonenumber, email probably make sense to be passed from the host for now. Collect provides an interface for setting these values so they can be accessed by any form with the corresponding preload.