Indicates whether a relation was marked readonly.

Motivation / Background

There's currently no documented API to check if a relation is readonly. Workarounds include loading the records and checking those for readonly? or using accessing the undocumented readonly_value.

A Relation could be marked readonly and used as an indication to what should be rendered. This change makes that use case simpler.

Detail

Adds ActiveRecord::Relation#readonly? which behaves the same as ActiveRecord::Base#readonly? for the contained records.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Unrelated changes should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.