Rename importer's plot ID fields

`OPENTREEMAP_ID_NUMBER` -> `OPENTREEMAP_PLOT_ID`
* Make it clear this is the Plot ID, not the Tree ID
* `PLOT` instead of `PLANTING_SITE` following existing fields like `PLOT_WIDTH` and `PLOT_HEIGHT`

`ORIG_ID_NUMBER` -> `EXTERNAL_ID_NUMBER`
* `EXTERNAL` clearer than `ORIG`
* Retain `NUMBER` since this is a user-populated field, to clarify what we expect

Note this changes no model attribute names, so effects are localized to the importer.
Create separate functions for committing plot data and tree data
Importer saves UDF values for trees
Add `transaction.atomic` to `commit_rows`
Merge pull request #1798 from RickMohr/save-udf-values

Importer saves user-defined field values for plots and trees