Show existing API Key on admin config form

Problem/Motivation

On the config form, the API Key field is blank, even if a key was saved previously. This is not a great developer experience. Even though it is stated in the description that the key is hidden, that is easily overlooked, causing confusion about how the SI module is able to send emails via SendGrid without an API Key configured.

Steps to reproduce

Enter an API Key. Revisit the config form. Note that the API Key field is blank.

Proposed resolution

Show the existing API Key saved previously. There's no reason to hide it. The config page requires admin-level permission. If the user could see the key when they entered it and saved it the first time, they should be able to see it on subsequent visits to that form.

Remaining tasks

User interface changes

API changes

Data model changes