[PNI] Create a BuyersGuideContentCategory snippet

[tbrlpld]

Description

Simple snippet to categorize the different kinds of articles.

Fields:

• title: CharField
• slug: SlugField

Acceptance criteria

• As a Wagtail editor, I can create content category snippets in Wagtail.
• As a translator, I can translate the content category title.
• As a Wagtail editor, I can not create two categories with the same slug and in the same locale. (Bug Unique together constraints including the locale cause IntregrityError wagtail/wagtail#8918 preventing this)
• As a Wagtail editor, I can create two categories with the same slug and different locale. (Bug Unique together constraints including the locale cause IntregrityError wagtail/wagtail#8918 preventing this)

[tbrlpld]

Since this is a PNI ticket, it won't go to staging. We can to QA once the PNI review app is ready.

[tbrlpld]

@cdanfon This is new ready for testing on the PNI preview app: https://foundation-pni-2022.herokuapp.com/en/privacynotincluded/

Login details in Slack: https://mozilla.slack.com/archives/C4M19QU2Z/p1659964009747769?thread_ts=1659961163.287079&cid=C4M19QU2Z

[tbrlpld]

Note: A bug in Wagtail unfortunately prevents the option from constraining the slug uniqueness only per locale. See also: wagtail/wagtail#8918. Because of this issue, translators will have to manually set a unique slug if the translated title matches any other language.

[cdanfon]

@tbrlpld

Couldn't satisfy AC #2

Steps to reproduce:

1. Tried to translate numerous snippets at once - it didn't translate
2. When trying to translate 1 snippet - 500 error came up

Aware that the last 2 ACs cannot be tested due to bug #8918 referenced by Tibor