Classification - assign sample/lab

Samples is VG only. Not sure if we'd allow lab changes in shariant for non admins.

This is a VG only feature, not Shariant.

So this should only apply where classification.lab.external = False. We can also probably limit it to users that have the lab head group permission so requires #1470

The sample needs to be an autocomplete as there are tens of thousands. The lab can be a simple drop down. Again make it external = False

If you change the sample, there should be an option to auto-populate sample fields? Warn if they are already populated.

Could show the auto populate preview like when you create the classification from variant