Rename extra_field field_type and extra_field_type to item_type and data_type

[ywarnier]

The extra_field-related tables are unnecessarily confusing because they have a "field_type" that identifies the type of value that should be stored in this extra field, and an "extra_field_type" that identifies the kind of item this extra_field relates to.

This should be clarified, renaming "field_type" to "data_type" (or "value_type") and renaming "extra_field_type" to "item_type".

Also, the term "value" in exta_field_values.value should be renamed to field_value because it is a reserved keyword in several DBMS: see https://www.petefreitag.com/tools/sql_reserved_words_checker/?word=value

[ywarnier]

This is linked to #2326

[stale[bot]]

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[christianbeeznest]

The PR is merged in master now, tested and confirmed.