Add autocomplete for [issue: and [work_item:
- 
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What does this MR do and why?
Adds autocomplete for [issue:] and [work_item:] references
References
- Closes: #535475 (closed)
Screenshots or screen recordings
| Before (feature disabled) | After (feature enabled) | 
|---|---|
| featureDisabled | featureEnabled | 
How to set up and validate locally
- Enable extensible_reference_filters feature flag (http://localhost:3000/rails/features/)
- Navigate to some issue
- Using the plain markdown editor type [issue: or [work_item:
MR acceptance checklist
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Related to #535475 (closed)
Edited  by Samuel Suikkanen