Etherpad must be embedded on the same tenant host

Original issue can be found at https://github.com/oaeproject/3akai-ux/issues/2974
Copy:
```
In my local environment I have my etherpad domain as: 0.etherpad.local, and my user tenant domain as localhost.

Safari will not accept Set-Cookie from 0.etherpad.local because it is of a different domain. This causes a problem with tenants have their own domain.
```

Potential fix:
Rather than having different hosts at [0-n].etherpad.oae.com we can expose the etherpad functionality at: tenant.oae.com/etherpad/[0-n]/*

This way all cookies originate from the same host and Safari should be able to handle them OK.