Improve email templates

Now that we have the routes for managing the requests, we need to update the templates of the emails to include useful information.

New OAuth client request from [user:name]
Add a link to the request edit page, so it's easy for them to moderate

Your OAuth client request has been approved
Add a link to the user client form and recommend them to update the secret and, optionally, update the label

Your OAuth client request has been rejected
The email should contain the rejection reason