Consider removing 255 character limit on rights field

[philipashlock]

The rights field is the only field with a character limit, but it's now being used for many different things:

• explanation of the accessLevel
• explanation for redactions
• explanation for non-public-domain licenses

Even using this to insert one of the FOIA exemption reasons for a redaction would exceed the 255 character limit.

Considerations:

• Should there still be a character limit, but perhaps a higher one?
• Should the rights field be more structured to facilitate these different uses? Dublin Core also splits these into more specific fields like accessRights

[rebeccawilliams]

I definitely think there should be guidance for filling out the rights field (see also #492) and long-term I'd like to see an accessRights field.

The brief FOIA descriptions on /redactions should be updated to be within the character limit.

[rebeccawilliams]

Adding the new-field tag, as this may require a new field for all the different uses we are currently trying to fit into rights.