Add 'join existing organisation' workflow for users without an invitation

[Joe-Heffer-Shef]

Problem

Part of #666. Currently the only way into an Organisation is to create a new one or be invited by an existing member. A user at an NHS Trust that already has an Organisation in SORT, but who has no invitation, has no way to discover that org and request to join it — so they create a duplicate.

Proposed workflow

• During/after sign-up, let a user search for an existing organisation (e.g. by name) instead of only offering "create a new organisation".
• If a matching organisation is found, let the user submit a request to join it, rather than joining immediately.
• Notify an existing ADMIN of that organisation (see OrganisationMembership roles in home/models.py) so they can approve or reject the request.
• On approval, create an OrganisationMembership for the requesting user; on rejection, notify the user.

Out of scope (tracked separately under #666)

• Detecting/merging organisations that are already duplicated.
• Any organisation switcher UI for users who belong to more than one organisation.
• Uniqueness/fuzzy-duplicate checks on Organisation.name at creation time.

Notes

This is the first piece of work planned for #666.