Support autocomplete suggestions in Content Editor

Problem to solve

Working with GFM provides access to helpful additions for things like user mentions and issue references. The Content Editor supports displaying these references in existing documents and you can write them manually as well. But sometimes you can't remember how to spell a username, or the issue number is not how you remember the relation. In Issues and Comments, you get suggestions for completing your query, scoped to your project/group/instance.

Proposal

Implement suggestions to match how they work in the issue editor.

Examples

Availability and Testing

Please ensure package-and-qa is run and passes in the MR. Depending on lower spec coverage, we might also benefit from testing this feature in an E2E flow, please include the counterpart SET (@jzhang_gl) in MR reviews.

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