Uploads: Check if strategy is enabled before use

Avoid returning bare API tracebacks from pixiv et al when login details
are not configured, and instead raise a generic error.

app/logical/sources/strategies/deviant_art.rb

@@ -77,6 +77,10 @@ module Sources
 
       FAVME = %r{\Ahttps?://(?:www\.)?fav\.me/d(?<base36_deviation_id>[a-z0-9]+)\z}i
 
+      def self.enabled?
+        Danbooru.config.deviantart_client_id.present? && Danbooru.config.deviantart_client_secret.present?
+      end
+
       def domains
         ["deviantart.net", "deviantart.com", "fav.me"]
       end

app/logical/sources/strategies/nico_seiga.rb

@@ -50,6 +50,10 @@ module Sources
 
       PROFILE_PAGE = %r{\Ahttps?://seiga\.nicovideo\.jp/user/illust/(?<artist_id>\d+)}i
 
+      def self.enabled?
+        Danbooru.config.nico_seiga_login.present? && Danbooru.config.nico_seiga_password.present?
+      end
+
       def domains
         ["nicoseiga.jp", "nicovideo.jp"]
       end

app/logical/sources/strategies/nijie.rb

@@ -64,6 +64,10 @@ module Sources
       DIR = %r{(?:\d+/)?(?:__rs_\w+/)?nijie_picture(?:/diff/main)?}
       IMAGE_URL = %r{#{IMAGE_BASE_URL}/#{DIR}/#{Regexp.union(FILENAME1, FILENAME2, FILENAME3)}\.\w+\z}i
 
+      def self.enabled?
+        Danbooru.config.nijie_login.present? && Danbooru.config.nijie_password.present?
+      end
+
       def domains
         ["nijie.info", "nijie.net"]
       end

app/logical/sources/strategies/pawoo.rb

@@ -24,6 +24,10 @@ module Sources::Strategies
     STATUS1 = %r{\A#{HOST}/web/statuses/(?<status_id>\d+)}
     STATUS2 = %r{\A#{NAMED_PROFILE}/(?<status_id>\d+)}
 
+    def self.enabled?
+      Danbooru.config.pawoo_client_id.present? && Danbooru.config.pawoo_client_secret.present?
+    end
+
     def domains
       ["pawoo.net"]
     end

app/logical/sources/strategies/pixiv.rb

@@ -65,6 +65,10 @@ module Sources
       STACC_PAGE = %r{\A#{WEB}/stacc/#{MONIKER}/?\z}i
       NOVEL_PAGE = %r{(?:\Ahttps?://www\.pixiv\.net/novel/show\.php\?id=(\d+))}
 
+      def self.enabled?
+        Danbooru.config.pixiv_login.present? && Danbooru.config.pixiv_password.present?
+      end
+
       def self.to_dtext(text)
         if text.nil?
           return nil