allow subsetting tabular files when starting SV inspector

[alpapan]

following on from another feature request,

I haven't fully thought this through but when creating a SV Inspector session, would it be possible to

• indicate to user that the file they uploaded is above the 30mb limit? currently we get a console error only
  is this limit on a uncompressed or compressed file (i use vcf.bgz) ? perhaps it would help if the limit was on number of rows? (that would require parsing however... but see below)

• allow the SV inspector screen to subset a file that is over 30mb, for example selecting a single reference scaffold?
  This is might make a bit of sense because the file is already fully fetched when the error occurs but not sure how much effort it would entail.

[cmdcolin]

as mentioned in #4689 there were a number of improvements that may make this issue unneeded. if it is still troublesome i might recommend splitting into these individual VCF's "outside of jbrowse"

[cmdcolin]

can follow up in #4689..