chore: ability to run Chatwoot as API only server (#2344)

- Ability to run chatwoot as an API only server
- Removes action cable testing gem since it is merged to rails 6

.env.example

@@ -143,6 +143,11 @@ USE_INBOX_AVATAR_FOR_BOT=true
 # maxmindb api key to use geoip2 service
 # IP_LOOKUP_API_KEY=
 
+
+## Running chatwoot as an API only server
+## setting this value to true will disable the frontend dashboard endpoints
+# CW_API_ONLY_SERVER=false
+
 ## Development Only Config
 # if you want to use letter_opener for local emails
 # LETTER_OPENER=true