Create a credentials model and form

[amberrignell]

Create a credentials model via the live view context editor which has a

name (string)
body (map/jsonb)
The name cannot be blank, and the body must at least be {} (empty map'ish)
Create a credentials form which contains

name
body (is a textarea in json for now)
We are limiting this to the bare minimum for now, so different types of credentials etc is beyond the scope of this issue.

implementation notes

1. text area input comes from the front-end as a string, but it must be saved in ecto as a map.

[amberrignell]

Hey team! Please add your planning poker estimate with ZenHub @elias-ba @taylordowns2000 @stuartc