Add update rule form (first pass)

[rhystmills]

What does this change?

This PR add an 'update rule' form to the rule manager. As part of this work, we've also added a GET endpoint for an individual rule via its ID.

How to test

With Composer credentials, run typerighter locally using ./script/start. Click on the pencil icon next to a rule. Can you edit the rule via the form as intended?

Accessibility

• [ ] Tested with screen reader
• [X] Navigable with keyboard
• [ ] Colour contrast passed
• [ ] The change doesn't use only colour to convey meaning