Replace UI flag note markdown editor

[zhouzhuojie]

Description

Replace mavon-editor with a simple markdown-it editor. With the minimum setup and a simple katex support.

Motivation and Context

Fix #370

How Has This Been Tested?

Manual tested the edit/view mode, and the history tracking.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-commenter]

Codecov Report

Merging #392 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #392   +/-   ##
=======================================
  Coverage   83.01%   83.01%           
=======================================
  Files          27       27           
  Lines        1755     1755           
=======================================
  Hits         1457     1457           
  Misses        214      214           
  Partials       84       84           

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