R1:81fa847bc52b

Make "JIRA Issues" field work better with noncredentialed accounts

Summary:
Currently, users get an error when making any changes to this field if they don't have a linked JIRA account.

Instead:

- We should only raise an error if they're trying to //add// issues, and only on the new issues. It's always fine to remove issues, and existing issues the author can't see are also fine.
- When we can't add things because there's no account (vs because there's a permissions error or they don't exist), raise…