Prevent SSL errors when attempting to send mail by retrieving assets from the filesystem

Since asset pipeline compilation was disabled in GitLab 9.3, the premailer gem would fallback to retrieving the assets via the network. In some cases, this would fail due SSL verification errors. To make it possible for the premailer gem to load assets directly from the filesystem, we need the working directory to have a link to the public directory where the assets are stored.

Closes gitlab-org/gitlab-ce#34231