Fix: Heading and paragraph colors not applied inside the cover block

[jorgefilipecosta]

Description

Fixes: #17706

The cover block had a rule to make paragraphs and headings inherit the color. This rule had higher specificity then what most themes use in their color classes so it made the colors selected by users not applicable inside the cover block.

This PR fixes the problem by not applying color rules if the class has-text-color is present.
Meanwhile, I discovered that the heading block is not adding this class when it should. I'm creating a parallel PR to address this issue.

How has this been tested?

I used the 2020 theme.
I added a cover block.
I added a paragraph inside it.
I selected the color "accent" in the color palette (first one).
I checked the post on the front end and verified the color was applied as expected (in master the color was white).

[ZebulanStanphill]

Not really important, but shouldn't this just be .wp-block-subhead:not(.has-text-color)?

[jorgefilipecosta]

I don't think so we also want to have selector p:not(.has-text-color) and h1:not(.has-text-color) etc... Having the selectors nested in this way ensures we apply the rule to all the selectors (not just .wp-block-subhead) that don't have a text color.

[ZebulanStanphill]

Oh, whoops. I misread the SCSS. 😛

[youknowriad]

Related to this #17704, why do we need to style h2? I guess this is not needed anymore right? (all the h2 references in this file). For example, you'll notice that it's impossible to "left align" and h2 inside a Cover.

[jorgefilipecosta]

Hi, @youknowriad, I answered this subject at #17704 (comment).
I think we should address it in a different PR.