Categories: Forms: Change 'Default' value from -1 to 0

[n7studios]

Summary

When defining the Form setting on Pages or Posts, key/value pairs have always been:

Value	Setting Label
-1	Default (i.e. inherit from Plugin Settings)
0	None (i.e. don't display a Form)
Form ID (integer)	Form Names (i.e. display the specified Form)

When defining the Form setting on a Category, key/value pairs were:

Value	Setting Label
default	None
Form ID (integer)	Form Names (i.e. display the specified Form)

The 'None' label was misleading, because selecting this choice would display the Form specified at Post or Plugin level. This PR changed the Form setting value and its label on a Category to better reflect this:

Value	Setting Label
0	Default (i.e. inherit from Post Settings)
Form ID (integer)	Form Names (i.e. display the specified Form)

However, this still isn't in line with Form settings on Pages and Posts, where Default is -1.

This PR resolves by matching the Category Form settings with the Post Form settings:

Value	Setting Label
-1	Default (i.e. inherit from Post Settings)
Form ID (integer)	Form Names (i.e. display the specified Form)

Automatic upgrade routines are included to change existing Category settings to reflect this underlying change.

Testing

• testCategoryDefaultSettingsMigratedToNewValueOnUpgrade: Test that Category Form Settings = Default have their underlying value changed from 0 to -1

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)