Missing global snippets API

Description

The API lacks support of the global snippets feature. The projects snippets are nice, but disabled per default, and thus users will more easily tend to publish snippets on global scope, than within projects.

Proposal

Update the /snippets endpoint which would help adding/deleting/modifying/listing all global snippets. Eventually, unifying with the projects snippets could be a good idea.

Links / references

I guess that's a case for the v9 API updates goals: #20070 (closed) (though if it could be backported to a future v8 that would be ♥)