Add template code highlighting

[mikaelGusse]

This PR adds the ability for the user interface to highlight code fragments that have been ignored in the comparison. The ignored parts are highlighted with a faint grey color to separate from the blue that is used for matching fragments. Feel free to suggest any improvements to the implementation or functionality!

[rien]

Code is looking good. However, I'm not convinced of the color of the highlight currently:

Current situation rgba(0, 0, 0, 0.05)

Since it is completely black with only 5% opacity, it looks blue-ish somehow - which is very similar to the match highlight color. One could mistakenly think this is just a "faint" match.

Suggested change rgba(220, 220, 220, 1)

By making the ignored highlight color opaque, it is better distinguishable from the background and also "overwrites" the match highlight color if it overlaps.

However, in both situations I don't know if it is clear for users who do not know this feature whether we are highlighting ignored code or not. @bmesuere what is your opinion on this?

[mikaelGusse]

I agree that this new color is certainly clearer. I was also wondering if there should be some "legend" which could explain what the different colors mean. It could be behind a question mark on the page somewhere perhaps? Or behind some hoverable element? Not sure about that

[rien]

I have been thinking about this issue, but cannot find a good satisfying solution.

To avoid stalling this PR I will approve & merge it and create a new issue to discuss these options further.

[bmesuere]

@rien sorry, I missed the mention somehow. Since the code is ignored, I would also try to (also) "fade" the code itself instead of only the background. Note that we will probably need a similar solution if/when we ignore comments.