Shorten Posts: Fix button/tags covering tab bar and search box/dropdown

[marcustyphoon]

Description

This prevents the UI elements added at the bottom of a Shorten Posts shortened post from covering the dashboard tab bar and search interface atop search pages when scrolled to a position in which one overlaps the other.

Resolves #1400.

Testing steps

• Confirm that the dashboard tab bar appears on top of the Shorten Posts expand button.
• Confirm that the dashboard tab bar appears on top of the Shorten Posts tags element.
• Confirm that the "top/latest/etc" selector atop a search page in the desktop layout appears on top of the Shorten Posts expand button.
• Confirm that the "top/latest/etc" selector atop a search page in the desktop layout appears on top of the Shorten Posts tags element.
• Confirm that the search bar atop a search page in the tablet layout appears on top of the Shorten Posts expand button.
• Confirm that the search bar atop a search page in the tablet layout appears on top of the Shorten Posts tags element.

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• Try to make an element from inside a post erroneously appear over the Shorten Posts expand button/tags element. Hopefully it can't; I didn't think of any way to.
• Confirm that Disable GIFs and Cleanfeed don't have unexpected behavior on shortened posts.