Expose settings via API

[robdefeo]

Is your feature request related to a problem? Please describe.
Settings are complex and important in Mailchain, its important to understand what settings are used to see the behavior of the client. A complete list of settings can be viewed using mailchain settings view and flags can be applied to include default values. It is not possible to view these settings through the API.

Describe the solution you'd like
Create an API endpoint that clients can query to view settings

• Create handler for settings
• Return configuration values
• Wire up hander as a GET request /api/settings