* app api: extract table editibility in metadata_utils into function

This wrapper determines if a pair of a schema and table are editable. We'd
like to use this logic elsewhere, so wrap into a function and test.
Signed-off-by: Dorian Johnson <2020@dorianj.net>

* app metadata_utils: minor code cleanup in _parse_editable_rule
Signed-off-by: Dorian Johnson <2020@dorianj.net>

* requirements.txt: add dataclasses

This allows using dataclasses for better type safety in 3.6. In 3.7 they
are built-in.
Signed-off-by: Dorian Johnson <2020@dorianj.net>

* server table metadata utils: add parser of table keys
Signed-off-by: Dorian Johnson <2020@dorianj.net>

* api put_table_description: test for editability
Signed-off-by: Dorian Johnson <2020@dorianj.net>

* api put_column_description: respect table editability
Signed-off-by: Dorian Johnson <2020@dorianj.net>