FEATURE: hide enable_personal_email_messages and min_trust_to_send_email_messages

These site settings are very hard to explain and only applicable for very
specific Discourse setups.

If an admin "enables staged users" which is used in support scenarios then
all staff can send "messages" directly to an "email".

The setting allows you to extend this to TL4 or any trust level.

Actual use case would be a support type setup with restricted staff. It is
quite rare so hiding this for now and re-evaluate keeping the setting in
2019

spec/components/topic_creator_spec.rb

@@ -126,7 +126,7 @@ describe TopicCreator do
       end
     end
 
-    context 'private message' do
+    context 'personal message' do
 
       context 'success cases' do
         before do
@@ -146,7 +146,7 @@ describe TopicCreator do
         end
 
         it "should be possible for a trusted user to send private messages via email" do
-          SiteSetting.expects(:enable_personal_email_messages).returns(true)
+          SiteSetting.enable_personal_email_messages = true
           SiteSetting.min_trust_to_send_email_messages = TrustLevel[1]
 
           expect(TopicCreator.create(user, Guardian.new(user), pm_to_email_valid_attrs)).to be_valid
@@ -155,7 +155,7 @@ describe TopicCreator do
 
       context 'failure cases' do
         it "should be rollback the changes when email is invalid" do
-          SiteSetting.expects(:enable_personal_email_messages).returns(true)
+          SiteSetting.enable_personal_email_messages = true
           SiteSetting.min_trust_to_send_email_messages = TrustLevel[1]
           attrs = pm_to_email_valid_attrs.dup
           attrs[:target_emails] = "t" * 256