Removed code duplication while importing datafiles

[IamEzio]

Context: Comment

This PR does the following:

1. Creates base.py and utils.py files in mathesar.imports directory.
2. Moves redundant code from csv.py and json.py to above created files.
3. Splits long methods in csv.py and json.py.

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  visible errors.

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[IamEzio]

@dmos62 Please take a look. Thanks!