[Backend] Step 3 API for trials

What does this MR do?

1. Adds an api to apply trial for a namespace. Internally, it invokes the api in subscription portal to apply the trial.

2. If the trial application succeeded, the user would be redirected to the Groups page (existing page), and shown a banner Trial has been applied. In order to identify the path as coming from Trial sign up, an additional boolean parameter trial is passed.

3. Renames select_namespace api to select

4. Creates lead in Marketo only if Trial User creation succeeded.

Required for trial sign up as part of https://gitlab.com/gitlab-org/gitlab-ee/issues/13233

CE back porting changes: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32936

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• 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