Buffer session manager

Buffer session manager
* Defines the location for the redirect response for invalid sessions.
* Default value is https://login.buffer.com in production and https://login.local.buffer.com in development
* Defines the internal URL to the session-service.
* Defaults to http://session-service-1.core in production and http://session-service-1:3000 in development.
Targets node and browsers with > 3% market share. See this tweet for more context:
https://twitter.com/jamiebuilds/status/1022568918949408768