Web IDE should respect .gitattribute language setting

GitLab supports setting the gitlab-language of a file using the .gitattributes file to force the syntax highlighting of blobs. We should also support this in the Web IDE

Proposal

Respect the gitlab-language attribute in the .gitattributes files

Links

https://docs.gitlab.com/ee/user/project/highlighting.html

https://docs.gitlab.com/ee/user/project/web_ide/index.html#comment-3869256360