Fixed HTML Validation error: "Saw U+0000 in stream." by using sanitize_text_field() where appropriate

Also fixed a few small typos and made some code a bit more efficient:
* As both panel classes were literally the same, merging them into one class with empty extended classes makes the code easier to maintain
* Moved the css to a file - again, easier to maintain
* Changed double quotes to single quotes if no parsing was done and added some line breaks to make the html code readable (needed this to figure out where the html validation error came from)