Creating new topics and collections within create modal

### As a content editor, I want to see content pages (department, info, service, guide, and official docs) separately from IA pages (TCs and topics) because I conceptualize content and IA separately.

Longer term solution for #2645: fancy create content modal that allows users to choose and create new TCs and topics.

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The issue names no files, tests, or entry points. Start by reviewing the existing create modal and the related issue #2645, then clarify the content-page versus IA-page flows and acceptance criteria before implementation; done should mean the agreed modal supports creating TCs and topics as specified.

Written by the indexing model from the issue text.