Battle plan 🏹🎯

Data modeling

[x] Create an ExternalOrganization model with all the fields from Organization
[x] Move foreign key configuration on Repository, Issue and others? to ExternalOrganization
[x] Create a migration that applies the model changes and copies all organizations from Organization to ExternalOrganization.
1. IDs are copied as it in a first step
2. Create a temporary column new_id and fill it with new UUID.
3. For all foreign keys, update value to new_id
4. Drop id and rename new_id to id.
5. Apply the foreign key schema changes
[x] Drop from Organization the orgs that are "sync-only" (no installation_id or not created from upgrade).

[x] In GitHub sync logic, make sure to use ExternalOrganization instead of Organization
[ ] Revamp Loops events

[x] Remove all the GitHub specific fields from Organization
[x] Rename name to slug (uniqueness field)
[x] Add an optional foreign key towards ExternalOrganization
- In the migration, make sure to relink the existing installed organizations (we can likely use installation_id to match them).
[x] Add an API to link an Organization to an ExternalOrganization after a successful GitHub installation
[x] Revamp Issues and Repository API to allow to be queried either by Organization or ExternalOrganization
[x] Remove the creator upgrade logic
[x] Revamp info fields (display_name, bio...)
[x] Revamp the API to create/edit/manage organization

User organizations: probably need a system to manage members.
- Can be done in a second step
- Either way, I don't think we need to keep the sync with GitHub org members
Pledge as organization features (fields like billing_email, total_monthly_spending_limit...): should we take this opportunity to remove this?