Change default email_in minimum trust to 2

Also, rename MinTrustToCreateTopicSetting to TrustLevelSetting, and apply it to more settings

app/models/trust_level_setting.rb

@@ -0,0 +1,18 @@
+require_dependency 'enum_site_setting'
+
+class TrustLevelSetting < EnumSiteSetting
+
+  def self.valid_value?(val)
+    valid_values.any? { |v| v.to_s == val.to_s }
+  end
+
+  def self.values
+    @values ||= valid_values.map {|x| {name: x.to_s, value: x} }
+  end
+
+  private
+
+  def self.valid_values
+    TrustLevel.levels.values.sort
+  end
+end