Alerting: Wire recording rules feature toggle into limits struct

[alexweav]

What is this feature?
Tiny PR that whether recording rules are allowed or not as a limit, from Validation's perspective.
Makes limits now depend on both features toggles and config. Update tests accordingly.

This was split out of another PR in order to make the diff cleaner.

Which issue(s) does this PR fix?:

n/a

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[stevesg]

LGTM