Rethink CircleCI workflow triggers

[josephjclark]

CircleCI is set up to run a suite of tests on branches.

At the moment it looks like it runs the build_and_test workflow on any non main branch.

One problem with this is that when contributors raise a PR from a form, and their fork is on main and targeting main, our CI won't run. This means we have to merge without automated tests - which is very very bad.

Another problem I think is that any branch which is pushed will have CI run on it. But this is likely to be a waste of computation on 80% of branches.

I think the setup we want is to run CircleCI on any open PR, regardless of what the branch is. Maybe even non-draft PRs, if we can do that.

What that really means is that CI tests only run on branches which are complete (or close to complete)