Metadata free edit is broken for admins

Refs https://gitlab.com/teklia/requests/-/issues/786

An admin user cannot currently edit the name of an existing metadata through the free form modal.

STR:

1. create an allowed metadata on a corpus
2. create a metadata matching that allowed metadata (as admin or normal user)
3. as an admin, edit the metadata using the free form modal (not the regular one)
4. the error this.selectedMetadata.value.trim is not a function pops up in the console