[cloud] Add org codehost connections and repositories

Created by: kopancek

Screenshots

How to test locally

1. start sourcegraph with sg start enterprise
2. Create a user, and modify the user tags in postgres:

UPDATE users SET tags = '{OrgsCode}' WHERE id = 1;

3. Modify env flags:

SOURCEGRAPHDOTCOM_MODE=true 
EXTSVC_CONFIG_ALLOW_EDITS=true

4. Restart sg to make use of the new env flags