Strategy registry

[hardyjosh]

Now that we aren't hardcoding .rain strategies directly into the webapp, we need a way to source them. We've spoken about this being an onchain registry, but probably there's a pragmatic interim solution we can use in the meantime.

[hardyjosh]

pragmatic interim solution suggested by @thedavidmeister is to simply have a file that is a list of urls (one per line), and those are the ones we show on the grid of strats.

we'd have a "registry file" hardcoded as the default (but users could override as they like, the same way they can paste a link to a strategy atm)

cc @hardingjam