Generate Annotated VCF/CSVs for download

Downloading a 1-2M record multi-sample VCF with annotations can take so long things time out

I used a few tricks in SACGF/variantgrid_com#86 but we should probably download it then stream a static file

Suggest:

* On VCF / Sample page - if there is no CachedGeneratedFile for the url, then display button
* Cicking button creates a CachedGeneratedFile + task
* This creates CSV/VCF server side behind a UUID
* Client side shows message about generation, and has spinner going
* Once done, updates CachedGeneratedFile
* Drop down a message that says “Creating VCF, download will automatically start if you ” -
* Cached generated file check

Current behavior:

view_vcf - cohort_grid_export (csv/vcf)
View_sample - sample_grid_export

can probably keep those URLs/