DEV: introduce new API to look up dynamic site setting

This removes all uses of both `send` and `public_send` from consumers of
SiteSetting and instead introduces a `get` helper for dynamic lookup

This leads to much cleaner and safer code long term as we are always explicit
to test that a site setting is really there before sending an arbitrary
string to the class

It also removes a couple of risky stubs from the auth provider test

lib/tasks/typepad.thor

@@ -12,7 +12,7 @@ class Typepad < Thor
 
     backup_settings = {}
     %w(email_domains_blacklist).each do |s|
-      backup_settings[s] = SiteSetting.send(s)
+      backup_settings[s] = SiteSetting.get(s)
     end
 
     user = User.where(username_lower: options[:post_as].downcase).first
@@ -26,7 +26,6 @@ class Typepad < Thor
       exit 1
     end
 
-    inside_block = true
     input = ""
 
     entries = []
@@ -103,7 +102,7 @@ class Typepad < Thor
   ensure
     RateLimiter.enable
     backup_settings.each do |s, v|
-      SiteSetting.send("#{s}=", v)
+      SiteSetting.set(s, v)
     end
   end