Inserting snippets is broken for newly created snippets

Summary

Newly created snippets can't be inserted. Applies only to gitlab.com (and soon to 14.0)

This issue is caused by GitLab switching to main branch name.

Steps to reproduce

1. Create a project snippet
2. Try to insert it with GitLab: Insert snippet command

What is the current bug behavior?

When calling API to get the snippet blob, you get 404

What is the expected correct behavior?

inserting the snippet

Relevant logs and/or screenshots

The extension makes a call like:

curl -H "PRIVATE-TOKEN: $TOKEN" "https://gitlab.com/api/v4/projects/20486274/snippets/2128747/files/master/blah.patch/raw"

(notice the master branch in the request)

We should instead make call to

curl -H "PRIVATE-TOKEN: $TOKEN" "https://gitlab.com/api/v4/projects/20486274/snippets/2128747/files/main/blah.patch/raw" 

(notice the main branch in the request)