add abbility to import/export inventory tables in json format

[ClemsP]

No description provided.

[pep8speaks]

Hello @ClemsP! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file openrepairplatform/inventory/admin.py:

Line 23:80: E501 line too long (87 > 79 characters)
Line 38:8: W291 trailing whitespace
Line 40:1: E302 expected 2 blank lines, found 1
Line 41:8: W291 trailing whitespace
Line 43:1: E302 expected 2 blank lines, found 1
Line 44:8: W291 trailing whitespace
Line 46:1: E302 expected 2 blank lines, found 1
Line 47:8: W291 trailing whitespace
Line 49:1: E302 expected 2 blank lines, found 1
Line 50:8: W291 trailing whitespace
Line 52:1: E305 expected 2 blank lines after class or function definition, found 1
Line 56:32: E231 missing whitespace after ','