[MM-340] Fix issue of receiving incorrect notification for various events in DM

[ayusht2810]

Summary

• Fixed issue of receiving incorrect notifications for various events in the DM
• Added logic to get notification when user is removed from assignee or reviewer
• Added logic to handle various events (description, title, etc.) updates

Ticket Link

• Fixes #469

Checklist

• [x] Completed dev testing
• [x] make test Ran test cases and ensured they are passing
• [x] make check-style Ran style check and ensured both webapp and server pass the checks

What to test?

• Update the list of reviewers or assignees from the MR author account and check for notification in the updated user accounts
• Update the list of reviewers or assignees from one account another from MR author and check for notification in the updated user and MR author accounts
• Check for the notification when MR title, description, label, milestone, branches, draft status of the MR

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 87.50000% with 18 lines in your changes missing coverage. Please review.

Project coverage is 35.67%. Comparing base (53b10f3) to head (16bbb47). Report is 24 commits behind head on master.

:exclamation: Current head 16bbb47 differs from pull request most recent head b0334c0

Please upload reports for the commit b0334c0 to get more accurate results.

Files	Patch %	Lines
server/webhook/merge_request.go	87.50%	12 Missing and 6 partials :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #471      +/-   ##
==========================================
+ Coverage   34.02%   35.67%   +1.65%     
==========================================
  Files          22       22              
  Lines        4021     4146     +125     
==========================================
+ Hits         1368     1479     +111     
- Misses       2515     2524       +9     
- Partials      138      143       +5     

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[ayusht2810]

@mickmister updated the function name. Other comments are pending on @hanzei and your reply.

[ayusht2810]

@hanzei Fixed the review fixes. Please re-review.