Record platform of sequencer in database rather than hard-coding as "Illumina"

The platform of the particular sequencer used for a run is output in the prep sheet. Currently, that value is hard-coded as `Illumina`, but since the lab also uses other kinds of sequencers (Nanopore, PacBio, etc), this is not adequate. This info needs to be recorded per-sequencer somewhere--preferably the database.

Note that this should almost certainly be done at the same time as #266 Move definition of number of lanes per sequencer into database from SequencingProcess and #430 Instrument_model is a standard vocabulary in EBI .