Issue #97, add explanation box - 2nd PR

[InfTkm]

Fixed #97 and deleted trailing white spaces according to Taskcluster's log.

https://youtu.be/xmY6yLb7gu4
Here is a demo. Hopefully, I understood correctly where the question mark box should be.

Explanation of what I have done:

Line 59 in zero_coverage_report.js: sets the variable needs_explanation to true for the filter, completely_uncovered.
Line 138 - 146 in base.html: add the tooltip if the filter needs explanation (i.e. completely_uncovered).
At the end of style.scss: I added the CSS for the tooltip (i.e. popup explanation box) using SASS format.
Let me know if you have any questions or feedback. Thanks.

[InfTkm]

I didn't know that I could update the commits in a PR without creating a new one. Sorry about the duplicated PR.

[La0]

Great work !

@marco-c I'm ok with the display code, even though the style is a bit overkill, it looks nice.
We could reduce the explanation text as it's simply a copypasta from an IRC discussion right now. WDYT ?

[La0]

And here is a link to the frontend from this build so you can test.

[marco-c]

Yes, definitely better to rewrite that:

For JavaScript, files with at least one function where all functions are uncovered. For C/C++/other languages, files which have no covered lines.

[La0]

@InfTkm Could you update the explanation text with the one mentionned above by Marco ?

[InfTkm]

@La0 Hi, the change is made, and you can see the result here.
Thanks.

[La0]

Thanks for this great contribution 👍