Move assets in preparation for introducing import_groups app

What does this MR do?

Related issue: #249151 (closed)

This MR makes two things:

• Moves import-related CSS to page-specific bundle
• Creates generic import_entities directory and puts import_project directory as subfolder of this one

As part of &4374 (closed) we will be introducing new import_groups app. Part of this app will be unified with existing import_projects, so I'm extracting step of moving things around for easier review of "content MR"

Screenshots (strongly suggested)

Before	After

(no change intended)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

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