P2: Extra bottom margin on callout boxes

[SergeyBiryukov]

On ​https://make.wordpress.org/polyglots/teams/?locale=ru_RU, there's a callout box at the bottom.

The box is supposed to have these styles:

.callout p {
    margin-bottom: 0.6em;
    margin-top: 0.5em;
}

They are inadvertently overridden with the theme's default article paragraph styles, resulting in extra bottom margin:

article.post p, article#post-new p, article.page p {
    margin: 0 0 1.2em 0;
}

See the screenshot.

[obenland]

In 6357:

Handbook: Take control of paragraph margins.

Makes sure that callout boxes are displayed correctly.

Fixes #3181.