[frontend] Fix border style on markdown

[RomuDeuxfois]

Proposed changes

• Extract react-mde style for our markdown class

Related issues

• Border style table doesn't work. #3924

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality
• I wrote test cases for the relevant uses case
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...