appview: implement pagination for pipelines index #521

on the whole this looks good, FilterBetween("row_num"...) is a bit magic because it is not possible to tell at a glance that row_num is a column of pipelines, but until we land on a good mechanism for a generic interface for paginating db responses, this is perfect.

thanks for working on this!

had to rename filter to Filter since i wanted to use it elsewhere, but otherwise mostly the same aside from the changes from #519

i think i'd like to rework this from scratch, unfortunately theres been plenty of codechurn around this part of the code over the past few weeks.