Factor out main cause area inference and update it to split on | in addition to /; also do second-order subcause identification

[vipulnaik]

The main cause area inference from cause area currently splits on / and picks the first piece. We want to also split on |; moreover, the logic should be factored out of the various files so that it's easy to change in one place.

Regarding the point about recursive subcause identification, that means that something like Animal welfare/factory farming/chicken should identify a main cause area of Animal welfare and should also identify a second-order subcause of Animal welfare/factory farming.