Add test to validate the maximum size of the `content` directory

Based on the suggestion: omnibus-gitlab!4726 (comment 459473659)

We are going to include part of the gitlab-docs repository to the Omnibus package. But the Omnibus package has a strict size limit. The new test will check if the content directory size does not exceed this limit.

I set 2MB limit for the content directory. It is an arbitrary number. I believe this limit is not too restrictive for adding/updating new JS libraries and images, but it will protect us from the significant content directory size increase.