Namerequest UI: implement amalgamation

[severinbeauvais]

This is to support amalgamation filings (numered and named).

Currently, the supported-amalgamation-entities feature flag does not allow any entity types to create an amalgamation in LEAR -- instead, they are redirected to COLIN.

When the subject FF allows some entity types, Namerequest UI will need to be enhanced to create a draft amalgamation filing and redirect to the temporary business page (same as we do for draft IAs/registrations).

• add code to create draft amalgamation filing
• change code to redirect to the temporary business page (depending on FF)
• verify that FF still blocks unsupported entity types

[severinbeauvais]

This is a duplicate of #18532. Closing.