Open Registration/Sign Up into a Team

### Description

Discussed [here](https://github.com/flowforge/flowforge-nr-dashboard/issues/56#issuecomment-1699290325)

Main driver for this would be users that have built a Dashboard on FlowFuse, they want to have in theory, anyone using that Dashboard, but want to ensure users are authenticated first to track activity and prevent malicious attacks.

As such, they want to be able to provide a visitor a sign up link, such that the visitor can sign up to access the Dashboard, with a FlowFuse account, and be auto-assigned the "Dashboard-Viewer" role upon sign-up into their FlowFuse team.

As such, this removes overheads of visitors having to reach out to the owner to reuqest an invite, and for the owner to then manually invite them with their email, etc.

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate

Start by reading the linked discussion in flowforge-nr-dashboard issue 56 and reviewing the FlowFuse account, team, and Dashboard access flows. Done means a visitor can use a sign-up link to create or access a FlowFuse account and is automatically assigned the "Dashboard-Viewer" role in the relevant team; no files or tests are named.

Written by the indexing model from the issue text.