Column selecting operations should use attnum

[mathemancer]

Problem

Currently, most column selection operations are using name or index to identify columns. This is brittle, and we've decided to change to using the attnum property of columns instead.

Proposed solution

Each function in mathesar/db/columns/operations/select.py should use attnum instead of column_index, and should properly use the table_oid, attnum pair to identify columns for its logic.

Additional context

We'll clearly need to modify any place where the functions in the above file are called as well.

[mathemancer]

N.B. the get_column_indexes_from_table function actually gets attnums, and should be renamed accordingly. The call should also be modified, of course.