feat: implement user tagging in incident comments

[Kunal-Darekar]

Description

This PR adds user tagging functionality to incident comments, allowing users to mention other team members in comments and send them notifications.

Features

• Users can mention others using the @ symbol in comments
• Mentions are visually highlighted in the comment thread
• Mentioned users receive real-time notifications
• Support for two mention formats:
  • Direct email mentions: @email@example.com
  • Display name with email: @display Name email@example.com
• Keyboard navigation in the mentions dropdown

Technical Implementation

• Added CommentMention model and database migration
• Created MentionsInput component with autocomplete
• Added notification system for mentions
• Added end-to-end test for the feature

Testing

• Manual testing of the mention functionality
• Added E2E test for creating an incident with mentions
  /fixes [🔨 Enhancement]: Allow tagging in Incident comment #2187
  /claim [🔨 Enhancement]: Allow tagging in Incident comment #2187

[vercel]

@Kunal-Darekar is attempting to deploy a commit to the KeepHQ Team on Vercel.

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[CLAassistant]

All committers have signed the CLA.

[skynetigor]

Hey!
Quick question — since we’re already using ReactQuill, could we integrate quill-mention to handle mentions instead of building a custom input from scratch?
It seems like a lighter approach, given that Quill is already part of our stack.

[skynetigor]

Hi @Kunal-Darekar
When I try to run your feature I get this error:

Could you please check if the feature works?

[Kunal-Darekar]

@skynetigor please have a look and let me know if anything else is needed.

[hemangsk]

@Kunal-Darekar Stop stealing screenshots please

[Kunal-Darekar]

@Kunal-Darekar Stop stealing screenshots please

@hemangsk , I sincerely apologize for including your PR screenshot in my submission. This was completely unintentional and an oversight on my part. I've removed the image and will be more careful with my PR documentation going forward. Thank you for bringing this to my attention.

[talboren]

@Kunal-Darekar i'm closing this in favor of this first PR that was opened: #4649. If it will be rejected, we'll re-open yours. Sorry for the inconvenience.