Dev env fixes

This gets the dev env working as it does currently (without Ansible). The following changes were required:

1. We mount the shared folder as the vagrant user, so we don't have to do everything as root. When we tried mounting it as www-data (in order for the symlink to `/var/www/securedrop`) our options were to give www-data a shell (confusing and not standard) or run everything as root, which is generally a bad and confusing idea.
2. We no longer use Apache to run the web server, instead using `./manage.py start` to do it. This uses Flask's built-in web server, which supports automatic reloading and provides convenient logging.
3. Various other changes to the Ansible configuration to support these two major changes. See diff.