fix dynamic branch pages

[flosell]

(related to flosell/lambdacd#76)

The issue was mainly compojure:

LambdaCDs ui-for returns a ring handler that responds to paths without a prefix, e.g. / to deliver the UI and /api for various endpoints the UI uses to get data.

compojure/GET just passes on the full path, so the LambdaCD ring-handler would receive requests for a path like /branch/master/api which it cannot resolve.
compojure/context gets rid of the prefix, fixing the issue.

The issue remaining was that each request would create a LambdaCD instance. I fixed this with a very crude cache-like approach: Mutable map from branch-name to pipeline-instance. If it exists return, otherwise create a new one and put it into the map.

It's probably not race-condition free (it could create branch-pages you never see) but it works as a proof of concept

[jonocodes]

Thanks!