uploads: temp disable Pixiv source strategy.

Pixiv API client is currently broken. Temporarily disable Pixiv source
strategy so direct image Pixiv uploads can still go through. The
posts.pixiv_id field will need to be backfilled later.

app/logical/sources/strategies.rb

@@ -2,7 +2,7 @@ module Sources
   module Strategies
     def self.all
       [
-        Strategies::Pixiv,
+        #Strategies::Pixiv,
         Strategies::Fanbox,
         Strategies::NicoSeiga,
         Strategies::Twitter,