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/static_file_handler.cr

@@ -119,7 +119,7 @@ module Kemal
 
       config = Kemal.config.serve_static
       original_path = context.request.path.not_nil!
-      request_path = URI.unescape(original_path)
+      request_path = URI.decode_www_form(original_path)
 
       # File path cannot contains '\0' (NUL) because all filesystem I know
       # don't accept '\0' character as file name.