[7644] Add product category validation

[richbrennan]

Closes #7644 and #7772

Adds validation to avoid:

• Categories with the same name
• Category being a descendent of itself
• Categories being nested too deep

Link to sample test page: https://foundation-s-feature-76-cx7ws7.herokuapp.com/cms/snippets/wagtailpages/buyersguideproductcategory/

Review site password: https://mozilla.slack.com/archives/CBKP2SG94/p1639070578008300

To test, go to the page above and try the following:

• Creating or editing a category to have the same name as an existing one
• Making a category a descendent of itself by setting it to be its own parent or a child of one if it's children
• Creating categories more than 2 levels deep

Checklist

Tests

• Is the code I'm adding covered by tests?