Cache node_modules during deploy

[chrismytton]

Now we're using timestamped deploys we want to avoid having to do a full npm install from scratch every time we deploy. This symlinks the node_modules directory into the vhost app directory before doing the npm install so modules are cached between deploys.

[struan]

Small comment but otherwise looks ok.

[chrismytton]

I've changed the post_deploy_actions.bash script to use absolute paths everywhere, so there's no ambiguous paths being used. I've also switched to copying the whole node_modules directory into each release rather than symlinking it because this makes the concept of rolling back to a previous release as simple as switching the symlink to point to an older version.

It also accounts for the (very unlikely) situation where there is a node_module that gets dynamically required at some point during a deploy, which could be missing if it's removed in the newer release.