Set up trial reassurances experiment & add simple version of sidebar [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Adds an experiment (and corresponding feature flag) for showing some simple reassurances to the user that they’re making a good choice by registering for GitLab & starting a trial. In the control experience, the page remains exactly as it was. In the candidate experience, the content area is divided into two columns on large and larger viewport widths. The left column houses the reassurance sidebar, and the right column contains the registration form as it already existed. On narrower viewport widths, only the registration form is shown (just as it is in the control experience).

For this MR, the sidebar is pretty much all text based. In the next iteration, it will feature a nice SVG illustration and actual company logos (also SVGs).

Screenshots

Before	After

Testing locally

To test this out locally, you will need to enable the corresponding feature flag:

Feature.enable(:trial_registration_with_reassurance)

You may also need to enable billing plans & trials:

ApplicationSetting.first.update(check_namespace_plan: true)

Then make sure you are logged out and navigate to the trial registration page: http://localhost:3000/-/trial_registrations/new

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related to #281027 (closed)