Ignore underscores in GFM auto-complete

What does this MR do and why?

This improves the auto-complete by ignoring underscores in the search, so typing underscores isn't mandatory, even if the emoji or quick action contains an underscore.

While it is not a fully fuzzy search (like in Slack or Discord), it makes using the auto complete a tiny bit nicer to use.

References

None.

Screenshots or screen recordings

Before	After	Slack (the ideal)

How to set up and validate locally

1. In a textarea (issuable description or note), type /addchild
2. See that it completes to /add_child
3. Repeat for emojis like :smallair to :airplane_small:

MR acceptance checklist

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