support intersection in primary contrasts

so when i saw where we landed w the de stepper ui, i realized we could fairly easily expand to support intersections w * in deseq design formulas.. wed have to make the primary contrasts choice support multiple sections, so radio -> checkboxes. then the design formula would iirc become something like:

~ covariate + primary1 * primary2

probably important to tell ppl when to use it, something like:

Interactions test whether the effect of one variable depends on another.
Use interactions only when you expect different biological effects across groups and have samples for all combinations of factors.