[GH-954]:Fixed issue 'DM comment notifications not working'

[Kshitij-Katiyar]

Summary

• Fixed the issue that users were not getting DM notifications when they are mentioned in a Jira issue comment.
• Updated the code to replace the jira username or email address with mattermost username.

Issue

• Issue #954

[codecov-commenter]

Codecov Report

Patch coverage: 62.50% and project coverage change: +0.01% 🎉

Comparison is base (1766506) 31.16% compared to head (68184a9) 31.17%.

❗ Current head 68184a9 differs from pull request most recent head 0ec335e. Consider uploading reports for the commit 0ec335e to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #961      +/-   ##
==========================================
+ Coverage   31.16%   31.17%   +0.01%     
==========================================
  Files          50       50              
  Lines        7409     7409              
==========================================
+ Hits         2309     2310       +1     
+ Misses       4909     4904       -5     
- Partials      191      195       +4     

Files Changed	Coverage Δ
server/utils.go	20.35% <62.50%> (+2.65%)	⬆️

... and 1 file with indirect coverage changes

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[mickmister]

LGTM 👍

[mickmister]

@DHaussermann Heads up that this is a customer reported issue. LGTM ready for QA review

[AayushChaudhary0001]

Tested and approved, LGTM

• This issue is tested on MM version-7.8.2 and Jira version-9.
• The comment notification in DM's seems to be working fine on mentioning any particular user.
• On changing the username and renaming it similar to the email ID, the comment notifications in DM's are working as per the requirement.