Multi pathogen support: backend submission

The submission flow for single pathogen organisms should remain the same.

For multi pathogen:

Metadata: as usual, nothing special here.

Sequences: We don't want the user to be forced to put the segment name here.

```
>key1
ACTG
```

For multiple segments:

```
>key1_segment1
ACTG
>key1_segment2
GTCA
```