Provide a URL to be able to sign up + become member of a private space

**Is your feature request related to a problem? Please describe.**

A client needs to have a public URL with which users can sign up AND be added to a private participatory space in the same action. It's the same action as generating an invite from /participatory_space_private_users/new but instead of just being able to send a personal invitation to an email, the system would provide an URL with a token that would enable the signup + addition to the private space.

**Describe the solution you'd like**

• To have a shareable URL that would enable a user to signup + be added to the private space.
• Each private space, in the screen where you can send a new invitation, would show the shareable URL so the admin can copy and share.
• The URL is the normal signup + a parameter with a token. This token identifies the destination space
• This token can be deleted or renewed (in case we want to stop the shared URL from working or having a new one)

**Describe alternatives you've considered**

Create a module that implements similar functionality.

**Does this issue could impact on users private data?**

Since the URL can be reused, there is the risk of the URL leaking and being used by users that are supposed not to. This can be mitigated if we provide a mechanism to renew the token in the URL: this way old URLs would stop working and you can create a new one.

**Funded by**

EIT

We are very tight on dates: we'd need to know ASAP if this would make sense in the core or if instead we implement this as a module.