Remove certain buttons in markdown toolbar

[chia-yh]

Summary:

Fixes #1178

Changes Made:

• markdown-toolbar.component.html: remove md-mention and md-ref buttons as they do not provide significant benefits in the context of a PE
• comment-editor.component.spec.ts: remove md-mention and md-ref from button tests

Proposed Commit Message:

Remove certain buttons in markdown toolbar

"Mention a user" and "Reference an issue, pull request or discussion" 
buttons in the markdown toolbar do not provide significant benefits in 
the context of a PE due to the lack of functionality and intended 
anonymity in the PE environment.

Removing these buttons avoids any potential confusion among users 
arising from the lack of support for these buttons.

Let's avoid any confusion by removing these buttons from the markdown 
toolbar.

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (94134a8) 54.63% compared to head (0e1c5ec) 54.63%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1196 +/- ## ======================================= Coverage 54.63% 54.63% ======================================= Files 101 101 Lines 2888 2888 Branches 535 535 ======================================= Hits 1578 1578 Misses 967 967 Partials 343 343 ```

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