Simplify testing followables flash messages

Checking the whole text is tricky because the text has a `<br>` tag, and
now Capybara doesn't normalize whitespace by default anymore.

Here are a couple more options we could use:

```
expect(page).to have_content strip_tags(message.gsub(/\s*<br>\s*/,"\n"))

expect(page).to have_content strip_tags(message), normalize_ws: true
```

But then developers would wonder why we're doing all this, and would
need an extra effort to fully understand the test.

Since the tests are only checking the presence of the flash message,
checking a relevant part of the test is enough, works with any version
of Capybara, and makes the test easy to follow.

spec/shared/features/followable.rb

@@ -55,14 +55,13 @@ shared_examples "followable" do |followable_class_name, followable_path, followa
     scenario "Should display new follower notice after user clicks on follow button", :js do
       user = create(:user)
       login_as(user)
-      create_notice_message = t("shared.followable.#{followable_class_name}.create.notice")
 
       visit send(followable_path, arguments)
       within "##{dom_id(followable)}" do
         click_link("Follow #{followable.model_name.human.downcase}")
       end
 
-      expect(page).to have_content strip_tags(create_notice_message)
+      expect(page).to have_content "We will notify you of changes as they occur"
     end
 
     scenario "Display unfollow button when user already following" do
@@ -90,14 +89,13 @@ shared_examples "followable" do |followable_class_name, followable_path, followa
     scenario "Should display destroy follower notice after user clicks on unfollow button", :js do
       user = create(:user, followables: [followable])
       login_as(user)
-      destroy_notice_message = t("shared.followable.#{followable_class_name}.destroy.notice")
 
       visit send(followable_path, arguments)
       within "##{dom_id(followable)}" do
         click_link("Unfollow #{followable.model_name.human.downcase}")
       end
 
-      expect(page).to have_content strip_tags(destroy_notice_message)
+      expect(page).to have_content "You will no longer receive notifications"
     end
   end
 end