Apply the same spam checks to public snippets (either personal snippets
that are public, or public snippets on public projects) as to issues on
public projects.
Adding the necessary API for the new /snippets Restful resource
added with this commit. Added a new Grape class `Snippets`, as
well as a `PersonalSnippet` entity.
Issue: #20042
Merge-Request: !6373
Signed-off-by: Guyzmo <guyzmo+gitlab+pub@m0g.net>