Linking a BCEID to a client number should include all locations

As a staff user I want treat a client number as a person in FTA when linking. Right now I choose a BCEID and then when choosing a client I see multiple records when a client has more than one location code.

I want to see one client number to choose, which will link all location codes for that number. I want to still see current location codes but not be able to pick any more granularly than selecting all of them or not at all. If new locations are added they should be linked automatically when the import job runs.

We're uber flexible in the current implementation, however if the business area agrees that `client number = person` then we can switch to what is proposed above to avoid confusion with staff, and them needing to manually link all locations for a given client, or needing to watch for updates.

This is a relatively straight forward change we can do in the UI without changing the API (with the exception of the auto linking in the import job, that one is an API task but not a huge one), and doesn't undo any steps we've taken towards agency agreements.

We should also display BCEID username.

Does this make sense @LisaMoore1 ? Had it on mind after a call with @ZoeSimon and had to dump it somewhere.

**Additional Context**
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**Acceptance Criteria**
- [ ] Given (Context), When (action carried out), Then (expected outcome)
- [ ] Given (Context), When (action carried out), Then (expected outcome)

The issue names no files, tests, or entry points. Start by tracing the BCEID/client-location linking UI and the import job, then confirm the business decision that a client number represents a person. Done means one selectable client number displays its current locations, future locations link during import, and the BCEID username is shown.

Written by the indexing model from the issue text.