Add blob viewer / syntax highlighting for LookML
### Problem to solve LookML files are rendered as plain text ([example](https://gitlab.com/meltano/looker/blob/350dc2ca083d019a09dd997252d92cd162c8ebdb/finance.model.lkml)). ### Proposal Given that we will be using Looker for gitlab.com (https://gitlab.com/gitlab-org/gitlab-ee/issues/6329), we should at least support syntax highlighting of these files. ### What does success look like, and how can we measure that? | Before | After | | --- | --- | | ![Screen_Shot_2018-06-14_at_10.51.35](/uploads/753a49a346167856bc2bd77b18331b3a/Screen_Shot_2018-06-14_at_10.51.35.png) | ![Screen_Shot_2018-06-14_at_10.52.04](/uploads/cdf1804fff79451cf31c2a543bafe314/Screen_Shot_2018-06-14_at_10.52.04.png) | ### Links / references https://docs.looker.com/data-modeling/learning-lookml/what-is-lookml
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