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/trending.cr

@@ -42,7 +42,7 @@ end
 def extract_plid(url)
   wrapper = HTTP::Params.parse(URI.parse(url).query.not_nil!)["bp"]
 
-  wrapper = URI.unescape(wrapper)
+  wrapper = URI.decode_www_form(wrapper)
   wrapper = Base64.decode(wrapper)
 
   # 0xe2 0x02 0x2e