Implement consistency across UI components for webhooks extension

[a-roberts]

Expected Behavior

We should have consistency e.g. modal fading background behaviour, text alignments, and the modal appearance and colours used (e.g. for the delete webhook modal and for the create access token modal).

Actual Behavior

vs

Steps to Reproduce the Problem

Install the extension, try to create a webhook and then try to delete a webhook.

[Megan-Wright]

/assign

[a-roberts]

Megan's changes are merged (#173), closing