Link Tax Forms to fiscal hosts

[Betree]

Related to #7346
Related to #6948

Tax forms are currently shared across fiscal hosts: if you have expenses on both OCF and OSC and submit your tax form, both hosts will have access to the same instance/file.

While this pattern simplifies the UX, it pauses a bunch of concerns:

1. Any action taken by one fiscal host (e.g., Ability to Invalidate a tax form #7134 or Ability to mark a tax form as "Not required" #6225) will be applied globally. Yet, marking a tax form as "Not required" on OCF shouldn't mark it as Not required for OSC.
2. When tax forms are manually marked as received, the first fiscal host may set a link that the second host is not allowed to see (that would be resolved by Ability for fiscal host admins to manually upload a tax form #7375)
3. Not having this relationship forces us to run complex queries to list the tax forms attached to a given host (see [Project] Tax Forms Admin #7346 (comment) for details).
4. Given how sensitive this information is, we should make sure we get the payee's explicit approval before sharing their tax information with the new host.
5. We've defined that, in the future, we may want to encrypt tax form information using a host-specific secret. This requires having one tax form instance per host.

Proposal

API

• Add a HostCollectiveId column to LegalDocuments
• Seed the values of HostCollectiveId, duplicating rows if needed (1 legal doc/payee/year/host)
• These "duplicated" documents can still share the same URL
• Update the tax form queries to group by host

UX

When a payee is required to submit a new tax form for Host B while they already submitted one to Host A, show them two options:

1. Copy already-submitted tax form (could be a list if multiple options)
2. Submit a new tax form