Fix manual entering of flow cells with NextSeq

[holtgrewe]

Should be fairly simple to update the regular expression as flow cell vendor IDs are fixed length, then slot length can be fixed to one and the separating undescore can become optional

• https://github.com/bihealth/digestiflow-server/blob/master/flowcells/forms.py#L41

FLOW_CELL_NAME_RE = (
    r"^(?P<date>\d{6,6})"
    r"_(?P<machine_name>[^_]+)"
    r"_(?P<run_no>\d+)"
    r"_(?P<slot>\w)"
    r"_?(?P<vendor_id>[^_]{9})"
    r"(_(?P<label>.+))?$"
)