Clean up the Create Project UI experiment

What does this MR do?

Original issue: #267488 (closed)

What's happening?:

Basically, this is a clean up of legacy project creation code (bye,bye tabs!) and we're moving code from experiment folder to vue_shared and removing non-relevant HAML parts

Why vue_shared?

This code will be reused to update new Group UI in #254931 (closed), so we're building a kind of abstraction here to reuse same things

While we're here (c)

We're also making small, yet important changes:

• utilize as much gitlab-ui classes as possible
• move specific CSS to page bundle so it will not be loaded elsewhere
• fix a couple of translations missing ProjectNew|.

Video

New_Project___GitLab__2_

(you can use https://gitlab.com/projects/new to view current state of the things). There are a couple intended padding/margin changes to fit in our grid

Does this MR meet the acceptance criteria?

Conformity

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• Documentation (if required)
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Availability 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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done