Add 'sub_type' as a valid config item back to EnsEMBL::Draw::GlyphSet::flat_file
Add 'sub_type' as a valid config item back to EnsEMBL::Draw::GlyphSet::flat_file

Since this change (https://github.com/Ensembl/ensembl-webcode/commit/f2f2654b8520b67fb5bea028406316708f676094),
the 'sub_type' config item has been replaced with 'type'. But there are still places
where 'sub_type' is used instead.

Fixes ENSEMBL-4628.

Ensembl/ensembl-webcode,Ensembl/ensembl-webcode,Ensembl/ensembl-webcode,Ensembl/ensembl-webcode,Ensembl/ensembl-webcode