Allow users to specify which syntax a file renders with in Sourcegraph UI

Created by: christinelovett

Feature request description

While viewing a file, allow a user to specify which syntax they want the file to render in. (Rather than having to explicitly set up in a Sourcegraph configuration location)

Is your feature request related to a problem? If so, please describe.

A user was viewing a shell script in a config file that isn't standardly named, and they want to see the shell script syntax highlighting instead of raw text.

Describe alternatives you've considered.

Manually configure what language the file should be highlighted as, based on the file extension pattern, in Site Config.

Additional context

The files in question are not scripts, they are config files, so it would be misleading to introduce a shebang at the top of them. The user came across sshrc config files named sshrc and rc. This may also apply to other rc files which tend to use sh syntax.

/cc @taylorsperry @jasongornall