Add GeoJSON support for viewing Snippets
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### Proposal
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Add support for the following:
1. GeoJSON support for viewing Snippets under _Projects_.
2. GeoJSON support for viewing Snippets under _Namespaces_.
### Use Case
I want to be able to _embed_ a GeoJSON Snippet in a web site (e.g. GitLab Pages).
### Background
#14134 added GeoJSON support for viewing files in a Project's _Repository_ but not files in a Project's _Snippets_ or a Namespace's _Snippets_.
#### GitHUB
Does render snippets (Gists) that are GeoJSON files.
* https://gist.github.com/MatthewSteen/0a5888f35bf724150b48aaa06c48734c
#### GitLAB
Does not render Snippets that are GeoJSON files.
* https://gitlab.com/-/snippets/3641952
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