Improve ignored code block highlighting in compare view

[rien]

The current compare view that shows matches now also shows ignored code blocks due to template code. Below shows an example where Dolos ignores code if it is prevalent in 30% of the submissions:

For some users it might not be immediately clear that the grey highlights signify ignored code blocks. We should present this in a way that is clear and accessible (catering for colorblindness as well).

One option is showing a legend clarifying the meaning of the colors.

Another option could be to show tooltips that reveal extra information about a block or multiple blocks when hovering over them:

• Whether this block is ignored or not, including the reason why (template code, automatically detected)
• The length of the current block (in kgrams)
• The number of submissions that match with this code block
• The underlying kgram data - however this can potentially become very large and cluttered.