Custom config fields/"form metadata"

The new "Form Metadata" activity described in #311 is awesome, and even more awesome would be the ability to define custom fields there so that data could be entered once at the app level and then included in all submissions. Examples could be a team name, region code, etc. This would prevent enumerators from having to fill out the same data over and over, thus reducing errors.

Things defined in this way would be accessible in forms via preloading, e.g. for a custom field with key `teamname`:

```

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Additional notes:

* Keys should be forced to be unique
* Custom fields should be included in existing "bulk configuration" functionality

Start with the Form Metadata activity described in #311 and inspect the existing bulk configuration functionality. Completion means supporting app-level custom fields with unique keys, making them available through the shown preload mechanism, and including them in bulk configuration.

Written by the indexing model from the issue text.