[data-import] feat: grey out non imported columns

[kaustubhdapurkar]

Describe your changes

Grey's out columns that were not imported during data import

chrome_X5WSlPXUZO.mp4

Issue ticket number and link

Issue Ticket Number - 507
Link - #507

Checklist before requesting a review

• I have read and understand the Contributions section
• Target branch is releaseCandidate and not master
• I have performed a self-review of my code
• I ran the unit tests and my PR does not break any tests
• I documented the changes I've made on the CHANGES.md and followed actual conventions
• I added a new section on how-to.md (optional)

[tprouvot]

Thanks for the PR @kaustubhdapurkar !
I'll take a look at it next week

[tprouvot]

@kaustubhdapurkar Few remarks concerning the design, I think that only grey out the header columns should be enough.
The fields name in the Field Mapping section are not greyed out.

Since this is a visual change and not all the users may want it, I would add a new option to enable this feature or not