Combine multiple .thumbnail > img styles

BS3 commits: 697a37e73f30bf2f5897b976ff38a427dd78212e
more grunt

BS3 commits: 147837efaa48696debf93c62e8d872f72d866117
remove unused `{{ page.base_url }}` variables, and fix the urls

BS3 commits: 9fbbce9b2ddfe46b87c2ef63f965c97c06a502bf