Problem Description's UX: make every code markdown section prettier and better

[Johnny850807]

In Judge-Girl/Judge-Girl#48, we make the markdown prettier by adjusting some css styles defaulted by the markdown-it library. This actually is good enough to read.

In the legacy judge girl, the pionteer did a great job on optimizing the code markdown section to enable the user to copy the text within it or show the line numbers. like:

While currently in the new version, we have achieved:

This issue aims to achieve more:

For each code markdown section, we have to provide

• copy button
• show line numbers
• better background theme (somehow in darkness)

features