DEV: Move image auto-grid back to experimental (#31296)

This feature was originally experimental. We made it baseline in #29572, however this was met with some pushback, so we have decided to make it experimental again.

The original PR couldn't be reverted, but this one basically does the same by cherry-picking all the changes back.

spec/system/composer_uploads_spec.rb

@@ -160,6 +160,8 @@ describe "Uploading files in the composer", type: :system do
   end
 
   context "when multiple images are uploaded" do
+    before { SiteSetting.experimental_auto_grid_images = true }
+
     it "automatically wraps images in [grid] tags on 3 or more images" do
       visit "/new-topic"
       expect(composer).to be_opened
@@ -225,6 +227,8 @@ describe "Uploading files in the composer", type: :system do
     end
 
     it "does not automatically wrap images in [grid] tags when setting is disabled" do
+      SiteSetting.experimental_auto_grid_images = false
+
       visit "/new-topic"
       expect(composer).to be_opened