Use terminology "snapshot" everywhere

Recently Stack decided to centralize on the term "snapshot".

Since they're synonyms it's not a huge deal, except that some links into Stack docs have broken, e.g.

stackage-server/templates/stackage-home.hamlet

Line 11 in 22977c3

    
           <p .stack-resolver-yaml><a href="https://docs.haskellstack.org/en/stable/GUIDE/#resolvers-and-changing-your-compiler-version">resolver</a>: #{toPathPiece name}

Plus it would just be tidy to follow suit and standardize on "snapshot".