Submit individual sequences via a form

[theosanderson]

We should allow users to submit sequences via a form which would just be the same as the edit page, but not prepopulated

[theosanderson]

(Prompted by @rneher 's issue - we should consider whether this should literally be the same as the edit page or if we should e.g. support a FASTA but then use the table for the metadata)

[fhennig]

I created a Slack thread to clarify how to integrate this form into the submission flow: https://loculus.slack.com/archives/C061ZQQM3N1/p1739872711926539

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At this point:

We want to add another option to submit a single sequence.

For now just use to form as is, the option to upload a FASTA file instead can be pushed to a subsequent PR.

For clarification: No backend changes will be needed - just craft pseudo files in the frontend and submit them to the backend.