Export and Import of Projects, update and refactor for cli and drf

[MyPyDavid]

Description

This PR updates and refactors the project export management command and adds a project import command.
Initially, It was GPT generated because many parts of the RDMO import functions were not easily re-usable for a management command. However the code has been re-evaluated and re-factored to fit better in the rdmo code base.
An extra added functionality is, the export and import of the project members (--include-memberships), it currently creates an extra JSON file with the memberships but it probably should be integrated to the export format (like XML). In another instance the users can then be created or retrieved via email addresses.
As a side note, for when using a socialaccount provider, that would require an extra "existing account adapter" that lets a social login be connected to the existing user via the verified email address for example.

This PR should aim to refactor the RDMO project export and import functions so that they can easily be re-used for these CLI commands and for the DRF endpoints.

CLI, management commands

The import_projects command:

--path DIR            Directory with exported project sub-folders (default: exports/).
--projects [ID ...]   Only import the listed project-ID folders.
--format FORMAT       Import plugin key to use (must be configured in ``settings.PROJECT_IMPORTS``).
--include-memberships 
[--create-new-users]       # optional: create users if they don't exist
[--as-user-id ID]          # request.user for plugin
[--as-username USERNAME]

and export_projects:

--export-mode {project,answers,view}   # default: project
--view-uri URI                         # required if --export-mode=view
--include-memberships                  # only valid in project mode
--projects [ID ...] --site-id --catalog-uri
--format xml|json|... --path DIR

Two methods render_member and render_member_user were added to the XML renderer.
The memberships can be exported to the xml and will look like this:

<memberships>
  <member>
    <role>owner|author|guest|…</role>
    <user>
      <id>…</id>
      <username>…</username>
      <first_name>…</first_name>
      <last_name>…</last_name>
      <full_name>…</full_name>
      <email>…</email>
    </user>
  </member>
  …
</memberships>

Try out locally:

# Export (project mode, XML, with memberships)
python manage.py export_projects \
  --export-mode project --include-memberships \
  --projects 1 --format xml --path /tmp/exports

# Import from exported directory (restore memberships, allow new users)
python manage.py import_projects \
  --dir /tmp/exports --projects 1 \
  --include-memberships --create-new-users

DRF, endpoints and actions

For exporting an existing project:

• api/v1/projects/projects/<pk>/export/<export_format>

For project creation with an import

• /api/v1/projects/projects/{id}/import-create-preview
• /api/v1/projects/projects/{id}/import-create-confirm/

For updating an existing project with an import

• /api/v1/projects/projects/{id}/import-update-preview
• /api/v1/projects/projects/{id}/import-update-confirm/

Related issues are #1277 #1357

[MyPyDavid]

• add memberships to project export (xml (no dc-uri), etc..)
  • Memberships can be exported by all users that can export a project (names and emails are already visible in the project UI).

• import projects with members can only be done by site managers (and higher)
• potentially create (silent invite aka directly add membership) or 'lookup' users for the memberships?

Import with memberships is only supported via the management command for now.

Other notes:
What about the settings.USER_API is it still needed?