Update to Crystal 0.31.0, resolve compiler deprecation warnings, update dependencies (#764)

* shard: update to crystal 0.31.0

Additionally, no longer use the Crystal "markdown" library which has
been removed from the Crystal stdlib in version 0.31.0.
See https://github.com/crystal-lang/crystal/pull/8115.

Also fix some deprecation warnings using the following commands:

    find . \( -type d -name .git -prune \) -o -type f -exec sed -i 's/URI\.escape/URI\.encode_www_form/g' "{}" \;
    find . \( -type d -name .git -prune \) -o -type f -exec sed -i 's/URI\.unescape/URI\.decode_www_form/g' "{}" \;
    sed -i 's/while \%pull\.kind \!\= \:end_object/until \%pull\.kind\.end_object\?/g' src/invidious/helpers/patch_mapping.cr

src/invidious/helpers/tokens.cr

@@ -69,7 +69,7 @@ end
 def validate_request(token, session, request, key, db, locale = nil)
   case token
   when String
-    token = JSON.parse(URI.unescape(token)).as_h
+    token = JSON.parse(URI.decode_www_form(token)).as_h
   when JSON::Any
     token = token.as_h
   when Nil