Add ability to import exported data

[MartijnR]

Fairly complex, but do-able with current export file that includes timestamps for each draft and final record

[MartijnR]

probably best to remove the timestamps and instead use a format that ODK Briefcase likes. We should use only instanceID to protect against duplicates and let the user decide. It would be nice to add a timestamp but if this throws of ODK Briefcase, it's probably not worth.

The only thing we should add to an exported record is whether it's status is 'draft'. Need to look into how ODK Collect marks draft records. If this is a problem we could [1] automatically switch any imported record to 'draft'.

Other considerations:

• all records in one file or per-record (or both)? Interface to pick records to import from list?
• support for media files (first check if Blobbuilder supports binary files (offline))
• support for importing into updated forms (no way to detect, so needs to be supported) - either validate a record during import and mark as draft if invalid or [1] always mark imported records as draft

[MartijnR]

makes a lot of sense to implement #307 at the same time since this will make exports actually useful