Reduce trials controller to 2 actions and use Services to drive logic

What does this MR do and why?

Furthers the goals of #393969 (closed) - see there for more detail.

• renames create_lead to create
• moves functionality of apply to create
• removes select action in favor of redirecting to new action with a trial step
• moves to a pure 2 step form setup

Screenshots or screen recordings

no change.

How to set up and validate locally

note basically trying to emulate feature spec steps in ee/spec/features/trials/saas/*

1. Setup to simulate SaaS and restart GDK
2. Create 2 new groups.
3. Visit trials new path as in http://gdk.test:3000/-/trials/new.
4. Fill out the form...
5. The rest would be really hard to emulate here w/o a fully setup local customersDot and is why we have the feature specs that haven't needed to change with this MR.
6. Namespace selection can be simulated by visiting http://gdk.test:3000/-/trials/new?step=trial

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #393969 (closed)