[HOLD for payment 2023-05-22] Add mentions auto-completion

[puneetlath]

As part of the mentions project, we will be adding auto-completion for mentions by using the @ symbol. We should break this up into at least two PRs:

1. Adding the generic AutoCompleteSuggestions component and refactoring the existing EmojiSuggestions to use the new AutoCompleteSuggestions component
2. Adding MentionAutoSuggestions which will also use the AutoCompleteSuggestions component

The details of the implementation are described in the doc here.

[MelvinBot]

Current assignee @puneetlath is eligible for the NewFeature assigner, not assigning anyone new.

[szebniok]

Hey, I'll be working on this.

[MelvinBot]

📣 @szebniok! 📣

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[puneetlath]

PR 1 is up!

[puneetlath]

Assigning @0xmiroslav as C+