permissions: records always in community feature

[Samk13]

❤️ Thank you for your contribution!

Description

Closes inveniosoftware/product-rdm#183

This PR adds an option to prevent deleting all communities in a record except for the last one, as our org requires at least one community per record. This feature is off by default and can be activated in your instance by adding:

RDM_ENSURE_RECORD_COMMUNITY_EXISTS = True

Then you will still be able to remove communities, but not the last one.

This PR is connected to this package we create, it prevents submitting any record without selecting a community from the API level. and prevents users from creating communities unless they have certain privileges that we set for the theme programmatically.
Future plans are to implement this as well with the master branch.

Checklist

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Third-party code

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[Samk13]

Opened this PR as a continuation of the previously closed PR #1283