feat: support local apps.json loading

This logic supports the following cases:

- Download a fresh apps.json and load it if missing
- Check if a local apps.json is old and get a fresh one if so
- Always save a local copy after downloading a fresh apps.json

The http.Head() call is faster than a http.Get() call (only carries back
respones headers) and aims to make the more general case more
performant: you have the latest copy of the apps.json and don't need to
download another one. This a direct port of our Bash implementation
logic.

Closes https://git.autonomic.zone/coop-cloud/go-abra/issues/9.

config/env.go

@@ -16,6 +16,7 @@ import (
 
 var ABRA_DIR = os.ExpandEnv("$HOME/.abra")
 var ABRA_SERVER_FOLDER = path.Join(ABRA_DIR, "servers")
+var APPS_JSON = path.Join(ABRA_DIR, "apps.json")
 
 // Type aliases to make code hints easier to understand
 type AppEnv = map[string]string