User's table configuration includes errant key `available_columns`

[jeremystretch]

NetBox version

v3.1.7

Python version

3.8

Steps to Reproduce

1. Open the sites list as an authenticated user
2. Add or remove a table column (to effect a custom configuration)
3. Inspect the UserConfig instance for the user in the NetBox shell:

>>> config = UserConfig.objects.get(user_id=$USERID)
>>> config.data['tables']['SiteTable']
{'columns': ['name', 'status', 'facility', 'region', 'tenant', 'tags'], 'available_columns': []}

Expected Behavior

The table preferences should include only the columns key.

Observed Behavior

The available_columns key is also present, and contains an empty list.

Although this has no known impact on NetBox's operation, this key should not appear in UserConfig data.

[jeremystretch]

@thatmattlove do you have any time available to look into this? It seems to stem from how the handleSubmit function in tableConfig.ts is processing the form data.

[jeremystretch]

There's a similar issue where, when a user clicks the "reset" button to remove the customization, columns gets set to an empty list in the UserConfig data. This doesn't currently cause a problem, however it will cause NetBox to ignore administratively-defined default preferences in release v3.2 (see #7759).