Make code editor font family follow theme

[KTibow]

Proposed change

Lets themes change font family of code editor with paper-font-common-code_-_font-family.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (thank you!)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Example configuration

frontend:
  themes:
    fontchange:
      paper-font-common-code_-_font-family: "Comic Sans MS, cursive, sans-serif"

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io