feat(website): Improve design and usability of edit and single sequence upload form

[fhennig]

As a follow up to #3710 we want to improve on the form, by making it possible for the user to upload sequence data as a file, instead of having to copy-paste it into a textbox. Also @theosanderson had some additional design suggestions for the form here: #3731

Design needs to consider that the same component is used in different places in the UI.

We want a file upload for each individual segment, i.e. for CCHF there will be three upload components. Not FASTA, just plain sequence data.

Form redesign (by Theo) for reference:

[theosanderson]

Yeah I'm not 100% sure if my design is better. Biggest thing is the file-upload thing - this would apply to the edit/revise page too. When I was preparing a revision for some metadata in a real sequence on PPX prod I found it super stressful because I was worried I might have accidentally inserted a character in the sequence - it would be great to make that impossible!