don't use PluginSetting.cached_plugin_setting outside plugin setting

fixes CNVS-31176

Also refactor a little so we're not querying the DB for the plugin
setting ~3 times per method call.

test plan:
- With the plugin disabled, observe that MathMan is not used for MathML
  or SVG generation.
- When plugin is enabled, observe that specific settings (whether or not
  to use for one or the other target (mml & svg); the base url) are
  respected.

Change-Id: Ie17942644e02088d967ad19a10b0f96f4cd57665
Reviewed-on: https://gerrit.instructure.com/88389
Tested-by: Jenkins
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: John Corrigan <jcorrigan@instructure.com>

lib/math_man.rb

@@ -1,29 +1,33 @@
 module MathMan
-  def self.base_url
-    plugin_setting.settings[:base_url].sub(/\/$/, '')
-  end
-
-  def self.enabled?
-    plugin_setting.present? && plugin_setting.enabled?
-  end
-
-  def self.plugin_setting
-    PluginSetting.cached_plugin_setting('mathman')
-  end
-
   def self.url_for(latex:, target:)
     "#{base_url}/#{target}?tex=#{latex}"
   end
 
   def self.use_for_mml?
-    enabled? && Canvas::Plugin.value_to_boolean(
-      plugin_setting.settings[:use_for_mml]
+    with_plugin_settings do |plugin_settings|
+      Canvas::Plugin.value_to_boolean(
+        plugin_settings[:use_for_mml]
       )
     end
+  end
 
   def self.use_for_svg?
-    enabled? && Canvas::Plugin.value_to_boolean(
-      plugin_setting.settings[:use_for_svg]
+    with_plugin_settings do |plugin_settings|
+      Canvas::Plugin.value_to_boolean(
+        plugin_settings[:use_for_svg]
       )
     end
   end
+
+  private
+  def self.base_url
+    with_plugin_settings do |plugin_settings|
+      plugin_settings[:base_url].sub(/\/$/, '')
+    end
+  end
+
+  def self.with_plugin_settings
+    plugin_settings = Canvas::Plugin.find(:mathman).settings
+    yield plugin_settings
+  end
+end