Do not show warning if CSV delimiter is not a comma

[matthew-white]

This PR changes what happens if something other than a comma is used as the CSV delimiter. A non-comma delimiter will continue to be shown in EntityHeaderErrors if the header row is invalid. However, it will not be returned in the warnings from parseCSV() or shown in EntityUploadWarnings.

What has been done to verify that this works as intended?

I updated tests to match the new behavior.

Before submitting this PR, please make sure you have:

• [x] run npm run test and npm run lint and confirmed all checks still pass OR confirm CircleCI build passes
• [x] verified that any code or assets from external sources are properly credited in comments or that everything is internally sourced

[matthew-white]

Ah yes, that's a test that was missing. There are still a number of those, and I've filed the issue #979 about that. But this is probably as good a time as any to add that missing test. I've just pushed a commit that tests that the EntityUploadHeaderErrors component use the delimiter from the CSV file.