Flesh out more web UI actions for new URI interface

Summary:
Ref T10748.

• Allow users to add new URIs by clicking a button instead of knowing a secret URI.
• Validate that URIs are actually valid URIs.
• Add enable/disable action and strings.

Test Plan:

• Created a new URI.
• Tried to create a nonsense URI, created a good URI.
• Enabled/disabled a URI.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10748

Differential Revision: https://secure.phabricator.com/D15825