Twenty Fifteen: Remove `esc_html()` from blog description.

The blog description gets esc_html()'d *into* the DB. It's also escaped because the filter for `get_bloginfo()` is set to 'display'.

see #30724.
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Twenty Fifteen: Don't escape translated strings.

Replace every unnecessary instance of `esc_html_x()` with just `_x()`.

see #30724.
Built from https://develop.svn.wordpress.org/trunk@30899


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Remove caching for `get_term_by()` calls.

The new cache group scheme causes term invalidation to be overly broad, so
that busting the cache for one term will bust the cache for all terms in the
taxonomy. We'll have another go at more focused use of the 'last_changed'
incrementor in a future release.

Reverts [29915], [30073], [30080], [30108], [30112].
See #21760.
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