Add UCR support

[Charl1996]

Ticket

Background:
This work adds support to the DET for exporting data sources from CommCareHQ using the api_get_ucr_data API endpoint.

Due to the generic nature of the DET, not much work was needed to make the UCR exports compatible.

One thing to note:
The datasources on HQ is stored in a custom table. This table does not have an id column and uses the doc_id column as the primary key. When HQ gives the datasource data it adds an id column (in addition to the doc_id column) which contains the same value as the doc_id column, but this new id column is the primary key in the DET database. If this is a problem I can change that, but that will require some additional changes to the DET.

[snopoke]

With this implementation the DET will not record any checkpoints (checkpoints currently only support the DatePaginator - see here). This means that each run of the DET will attempt to re-export the full UCR table.

To support checkpoints I think we could do one of 2 things:

1. (preferred) Update the checkpoint to allow storing the 'cursor' value
2. Use our knowledge of the internals of the API create a new 'DateCursorPaginator' which extracts the date and doc ID from the cursor to store in the checkpoint.

Note that we don't currently use 'checkpoint.last_doc_id' at all. If we want to use it in pagination we'll need to make some other code changes to pass it (or the whole checkpoint) through.

[Charl1996]

@snopoke Thanks for highlighting that (and sorry for only coming back to you now).

I'm almost done adding the checkpoint support (draft PR). For now, though, I'm officially opening this PR for review.

[snopoke]

FYI, #224 should fix the build