Rethink the project snippets visibility levels within their project's visibility level

https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/1946 fixed an issue with private snippets in a public project being viewable by anyone but it isn't perfect and doesn't handle well the following cases:

• Public snippets on a private project doesn't make sense
• If your a team member on a private project, you should have access to all of its snippets (including its internal ones).
• Internal snippet on a private project should be visible to any team member.
• What about external team members?

This should probably be tackled as follow:

• public project: in that case, snippets visibility are respected
• internal project: public snippets should be considered as internal, other snippets visibility are respected
• private project: all snippets should be considered as private

Team members, including external team members can see any snippets of project it has access to.