Support retrieving and persisting files in server

[javierluraschi]

In order to allow users to edit code in the designer... or at least add code to the file they are editing when a control is dragged... we need to add support for save and persist files.

I would propose we support:

POST /putfile
{
  path: 'path to modify',
  contents: 'file contents'
}

POST /getfile
{
  path: 'path to retrieve'
}

We could also make /getfile a GET which is not compatible with what we have in our cloud, but would be OK to change.