[Account] Implement shared account

[rmontagnin]

Referencing Simon's tweet, I do agree that having the ability to create a shared account would be great.

Ideally, this can be done in two different ways:

1. Using the authz module.
In this case, the owner of the account would give other users the permissions to post on their behalf. This would maintain the granter as the owner and author of all articles.

2. Using the susbspace sections' permissions.
In this case, the owner of a subspace section (that represents the space inside which articles are placed) could grant other users the permission to write there. The section would then be the same as a Medium publication where different users can publish posts by remaining the authors, but all those posts are grouped in the same place.

The best thing would be to allow for both options, describing to the user the differences and letting them choose the best option for their case.