FEATURE: Allow email image embed with secure media (#10563)

This PR introduces a few important changes to secure media redaction in emails. First of all, two new site settings have been introduced:

* `secure_media_allow_embed_images_in_emails`: If enabled we will embed secure images in emails instead of redacting them.
* `secure_media_max_email_embed_image_size_kb`: The cap to the size of the secure image we will embed, defaulting to 1mb, so the email does not become too big. Max is 10mb. Works in tandem with `email_total_attachment_size_limit_kb`.

`Email::Sender` will now attach images to the email based on these settings. The sender will also call `inline_secure_images` in `Email::Styles` after secure media is redacted and attachments are added to replace redaction messages with attached images. I went with attachment and `cid` URLs because base64 image support is _still_ flaky in email clients.

All redaction of secure media is now handled in `Email::Styles` and calls out to `PrettyText.strip_secure_media` to do the actual stripping and replacing with placeholders. `app/mailers/group_smtp_mailer.rb` and `app/mailers/user_notifications.rb` no longer do any stripping because they are earlier in the pipeline than `Email::Styles`.

Finally the redaction notice has been restyled and includes a link to the media that the user can click, which will show it to them if they have the necessary permissions.

![image](https://user-images.githubusercontent.com/920448/92341012-b9a2c380-f0ff-11ea-860e-b376b4528357.png)

spec/mailers/user_notifications_spec.rb

@@ -699,50 +699,6 @@ describe UserNotifications do
     expect(mail.body.to_s).to match(I18n.t("user_notifications.reached_limit", count: 2))
   end
 
-  describe "secure media" do
-    let(:video_upload) { Fabricate(:upload, extension: "mov") }
-    let(:user) { Fabricate(:user) }
-    let(:post) { Fabricate(:post) }
-
-    before do
-      SiteSetting.s3_upload_bucket = "some-bucket-on-s3"
-      SiteSetting.s3_access_key_id = "s3-access-key-id"
-      SiteSetting.s3_secret_access_key = "s3-secret-access-key"
-      SiteSetting.s3_cdn_url = "https://s3.cdn.com"
-      SiteSetting.enable_s3_uploads = true
-      SiteSetting.secure_media = true
-      SiteSetting.login_required = true
-
-      video_upload.update!(url: "#{SiteSetting.s3_cdn_url}/#{Discourse.store.get_path_for_upload(video_upload)}")
-      user.email_logs.create!(
-        email_type: 'blah',
-        to_address: user.email,
-        user_id: user.id
-      )
-    end
-
-    it "replaces secure audio/video with placeholder" do
-      reply = Fabricate(:post, topic_id: post.topic_id, raw: "Video: #{video_upload.url}")
-
-      notification = Fabricate(
-        :notification,
-        topic_id: post.topic_id,
-        post_number: reply.post_number,
-        user: post.user,
-        data: { original_username: 'bob' }.to_json
-      )
-
-      mail = UserNotifications.user_replied(
-        user,
-        post: reply,
-        notification_type: notification.notification_type,
-        notification_data_hash: notification.data_hash
-      )
-
-      expect(mail.body.to_s).to match(I18n.t("emails.secure_media_placeholder"))
-    end
-  end
-
   def expects_build_with(condition)
     UserNotifications.any_instance.expects(:build_email).with(user.email, condition)
     mailer = UserNotifications.public_send(