Show a warning when Encounter.submitterID field is invalid during bulk import

[goddesswarship]

Expected behavior
When a user bulk imports a spreadsheet and there is an invalid entry in the Encounter.submitterID field (username doesn't exist or capital letters are used when it's all in lowercase in User Management), an error should appear in the bulk import preview.

Current behavior
When a user bulk imports a spreadsheet, the preview page shows Encounter.submitterID column in green whether or not the username is entered exactly as it appears in User Management (or even if the user exists in that Wildbook).

This leaves all of the uploaded encounters unassigned to a user.

This behavior was tested and observed in IoT and GiraffeSpotter.

Community link
https://community.wildme.org/t/unable-to-assign-encounters/2197