Fix length mismatch when copying code

[luin]

CodeBlockContainer#html() generates duplicated \n when deals with empty lines. See the test plan for reproduction steps.

This PR added a fix for that, also fixed a related issue that copying & pasting code block doesn't keep the highlighting language.

Test Plan

1. Run npm start
2. Visit http://localhost:9000/standalone/full/
3. Copy the following code block to the editor:

start





end

4. Add some plain text after the code block. See the screenshot below.

5. Copy the code block in the editor and paste after the plain text.
6. Make sure the pasted code block has the same content as the original one.
7. Set a highlighting language after step 3 and try again. Make sure the pasted code block keeps the language setting.