Convert the discover page use of hand raise lead to use new method

What does this MR do and why?

Convert the discover page use of hand raise lead to use new method

• next step in our conversion effort of the hand raise lead to use our new way to launch.
• first haml invocation here, so a bit more setup code for it.
• also moved the discovers helper to match the controller nameing to match rails conventions a bit more closely.
• broken off from larger: !151601 (closed)
• see #460511 (closed)

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Screenshots or screen recordings

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How to set up and validate locally

note: covered by this spec ee/spec/features/groups/discovers/hand_raise_lead_spec.rb

1. Setup gdk to simulate SaaS - https://docs.gitlab.com/ee/development/ee_features.html#simulate-a-saas-instance and restart GDK
2. Enable feature flag by running echo "Feature.enable :trial_discover_page" | gdk rails c
3. create a group
4. In the gitlab admin UI, edit the group to be on an ultimate trial
5. In rails console, change the trial, trial start and end date for the group

   Group.last.gitlab_subscription.update_columns(trial_starts_on: Date.today, trial_ends_on: 30.days.from_now, trial: true)

6. Add return to the very first line of authorize_discover_page. I am not sure why enabling the feature flag isn't working to allow this through, but it isn't important for this test.
7. Navigate to http://gdk.test:3000/groups/<group_name>/-/discover
8. Once on the discovers page click the 'Contact sales' button at top and bottom of page to launch the modal

Related to #460511 (closed)