Changes css style specificity to 0

[majapw]

While having specificity be quite high is a necessity in having react-with-styles-interface-css be fully operable, it is not actually necessary (with one small tweak) for react-dates itself. In order to have our exported css file be less of a trash-fire, I am reducing the specificity to zero.

to: @ljharb @lencioni

[lencioni]

I'm not really familiar with this registerMaxSpecificity function, so I'll have to leave the stamp up to someone else I think.

[lencioni]

Is this related?

[majapw]

Yes! So basically specificity is what generates something like:

.CalendarDay, 
.CalendarDay_1.CalendarDay_1,
.CalendarDay_2.CalendarDay_2.CalendarDay_2,

and then the appropriate class name is applied based on the position styles.CalendarDay takes in the css call.

In this particular method, we were relying on a particular order of hoveredSpan vs. selected to do styling overrides. By not applying the hoveredSpan class to selected days, we get around this necessity. As a result, this allows us to remove the specificity requirement and make the css file we ship with react-dates much smaller.

In general, we'd recommend using a high specificity value with RWS-ICSS (it defaults to 20), but in this small package, it's not strictly necessary.

[ljharb]

So then this change means that selected days no longer have a hover effect?

[majapw]

There is still a hover effect on selected days as defined by https://github.com/airbnb/react-dates/blob/4a8d379a2d23c1e0c265545b8ac7f2e78ab3a607/src/components/CalendarDay.jsx#L259-L263. The hovered span styling are completely different and before we were relying on the ordering of styles to apply selected over hovered. However, this change means that those styles will never be applied at the same time which will solve this issue.

[coveralls]

Coverage remained the same at 86.733% when pulling 4a8d379 on clean-up-exported-css into 32c536c on master.