Syntax highlighter uses classes, style-src locked down

[RobinDaugherty]

• Use CSS classes to provide code syntax highlighting style.
• That gets rid of the last use of style attributes, so add a nonce to the style tag and lock down the style-src Content Security Policy.
• Also fix the attempted load of a favicon. We never referred to one in Better Erors, but the browser attempts to load /favicon.ico by default. So by providing a blank one and allowing data: img-src in the CSP, we can stop the browser from complaining about it. (It might be nice to actually have a favicon representing Better Errors, but it doesn't seem very important right now.)

[coveralls]

Pull Request Test Coverage Report for Build 418956541

• 6 of 6 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.798%

---

Totals
Change from base Build 418955866:	0.0%
Covered Lines:	514
Relevant Lines:	531

---

💛 - Coveralls