Default to read only field on import if only one file format is defined

[matthewhegarty]

Problem

Closes #1682

If only one file format is defined in IMPORT_FORMATS, then display a read only field:

settings.py:

from import_export.formats.base_formats import CSV, XLSX
IMPORT_FORMATS = [CSV]

This makes the import process consistent with the export process.

Solution

Update the form to add extra logic to handle read-only fields.
Update the import.html template.

Acceptance Criteria

• Integration tests
• Manual testing

[pokken-magic]

Nice quality of life improvement. my code comment is not a blocker.

[pokken-magic]

This is a super minor nit, but I think this if chain could be a little cleaner using a guard clause pattern:

if not import_formats: 
    raise ValueError("Invalid import formats list"  
if len(input_formats) == 1:
   <do label stuff>  
if len(input_formats) >= 1:
   <do guess_format stuff> 

It looks to me like the ".choices" might be redundant (or only used in input_formats >1 clause) so could be moved inside that ifcheck as well--but I'm not sure there.

[matthewhegarty]

You're right. I looked into this and decided that the forms module could do with a refactor to avoid duplication. This means that I have added a few more changes so that the forms classes are standardised.

I'd be grateful if you could take a look.

[pokken-magic]

This looks like a solid quality of life! My code comment is non-blocking

[pokken-magic]

This refactor looks good to me. Lots of moving bits so possible to miss something, but hey, tests :) Good work!