We're not fully standardising here: different contexts can store their range state however
they like. What we are standardising on is that now the view is always responsible for
highlighting the selected lines, meaning the context/controller needs to tell the view
where the range start is.
Two convenient benefits from this change:
1) we no longer need bespoke code in integration tests for asserting on selected lines because
we can just ask the view
2) line selection in staging/patch-building/merge-conflicts views now look the same as in
list views i.e. the highlight applies to the whole line (including trailing space)
I also noticed a bug with merge conflicts not rendering the selection on focus though I suspect
it wasn't a bug with any real consequences when the view wasn't displaying the selection.
I'm going to scrap the selectedRangeBgColor config and just let it use the single line
background color. Hopefully nobody cares, but there's really no need for an extra config.
We've been sometimes using lo and sometimes using my slices package, and we need to pick one
for consistency. Lo is more extensive and better maintained so we're going with that.
My slices package was a superset of go's own slices package so in some places I've just used
the official one (the methods were just wrappers anyway).
I've also moved the remaining methods into the utils package.
