[MM-17] Fix mention notifications and label/milestone RHS display

[raghavaggarwal2308]

Summary

1. Mention notifications not working.
2. Labels and milestones not displayed in the RHS.

What to test?

• Labels and milestones are properly displayed in the RHS.
• Getting DM notifications for mentions.

Steps to reproduce:

• Labels and milestones:

  1. Connect your GitHub account.
  2. Create a PR on Github and add labels/milestones to it.
  3. Open RHS for open PRs on Mattermost.

• DM notification:

  1. Connect your GitHub account.
  2. Create a webhook.
  3. You should get notified in GitHub bot DM when someone else mentions/assigns you in a PR/Issue.

Ticket Link

N/A

[codecov-commenter]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (5aa2450) 15.79% compared to head (ea87542) 15.78%.

Files	Patch %	Lines
server/plugin/plugin.go	0.00%	6 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #721 +/- ## ========================================== - Coverage 15.79% 15.78% -0.01% ========================================== Files 15 15 Lines 5767 5770 +3 ========================================== Hits 911 911 - Misses 4814 4817 +3 Partials 42 42 ```

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[raghavaggarwal2308]

@hanzei Gentle reminder for re-review

[raghavaggarwal2308]

@mickmister Fixed the comments added by you