feat(website): support column remapping for uploaded metadata files

[fhennig]

As a user I want to be able to upload metadata files without having to edit them, even if the columns do not match the metadata template given by Loculus. Instead I want to be able to select which column in my file to map to which column in the metadata template. I expect the UI to be smart about the column mapping, and automatically map columns with identical names to each other.

Relevant PoC: #3426

I want to know which columns I have to supply, and which are optional. This requires #3430 (not blocking though I think)

Some open questions (not blocking):

• Which way should the mapping go? given file columns -> expected columns or the other way round? (This can probably be changed easily, so possibly no need to discuss a lot; just pick one way for now)
• If the user discards the file, should we try to keep the mapping around? (Might be annoying to edit data, the file disappears, I have to select the mapping all over again)
  • Idea: Maybe keep the file handle around, detect if the file changed, make the user re-submit it, but cache the mapping as good as possible (just on the site - gone if page is refreshed)

[theosanderson]

It would be great as part of this to automatically recognise the displayName as a default mapping if the technical name is not present