CiviContribute Import : Clarify user interface for Insert vs. Update mode and require unique reference (matching field) for Update mode

Contribution import functions in EITHER "insert new contribution records" OR "update existing contribution records" mode. This is not clear from the current UI. Also, Update mode needs to have a mapped "unique reference" field in order to properly link import file rows to the corresponding existing contribution record (this includes the ability to update custom fields properly).

1. Modify the step 1 form as follows:

• Change field label from "On duplicate entries" to "Import mode"
• This field should have TWO radio buttons (insert is default value):
  [ ] Insert new contributions
  [ ] Update existing contributions

2. If "Update" mode is selected, in step 2, add '(match to contribution record)' to the option labels for Contribution ID, Invoice ID and Trxn ID. And add a form rule that user MUST select one of these 3 possible unique reference fields in this mode.

Verify that custom contribution field values in the import file are saved properly to the DB in BOTH insert and update modes.

— Original Post from Brian S ----
If you import contributions and select the "update" option, custom field import data is ignored. When "update" is not selected, custom data is imported correctly.