Twenty Seventeen: Fix incorrect `$content_width` value in theme

This addresses a major bug. Incorrectly setting the `$content_width` causes media embeds to end up with the wrong aspect ratio, among other issues. This fix uses `template_redirect`,  to ensure conditional theme tags can be used. It also defines a default value at `after_theme_setup` so that plugins have something to work with at `init`.

Props sstoqnov, laurelfulford, obenland.

Fixes #39272.

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git-svn-id: https://core.svn.wordpress.org/trunk@39575 1a063a9b-81f0-0310-95a4-ce76da25c4cd