If a user provides an annotated dictionary, the app ignores it but keeps the annotations

[surchs]

We do not yet support uploading an already annotated data dictionary (see #601). However, if a user regardless uploads an already annotated data dictionary the app will happily take it. This leads to some very confusing states:

• if I provide a completely annotated data dictionary 🤷 then the visual state of the app will tell me nothing is annotated yet, but my download will include all my existing annotations
• if I provide an invalid data dictionary then again, the UI looks empty, but when I come to the download page and my data dictionary is validated, the validation will fail. Unless I guess what's going on, I cannot fix that

So in short: the state of the data dictionary in the app and the visual UI do not correspond to each other, and that's quite bad.
We have two options:

1. Teach the app to load state from a data dictionary ([ENH] Let user upload an already annotated data dictionary #601)
2. Put a big red warning on the first page that says: you cannot load a data dictionary with neurobagel annotations

I lean towards 2 because we will likely refactor this soon

TODO:

• when existing data dictionary is loaded, strip away / rest the "Annotations" part
• big warning on Home page near the "data dictionary" upload button to say "cannot reuse annotations"
• test that existing annotations do not make it through the tool, e.g. no unintentional annotations