404 page improvements

[benlk]

Changes

• When used on a search page, the search form partial will display the search query. (Does not affect the header search; that does not use searchform.php
• The 404 page shows an optional text message supporting basic HTML under the search form, which is set in the theme options under Advanced options.
• partials/content-not-found displays the 404'd URL in a <code> element before the search box if the page is a 404. Otherwise, it continues to display "Apologies, but no results were found. Perhaps searching will help.'"

Here's an example with a successful search, showing the query in the search box:

An unsuccessful search, showing a different text suggesting running a different search:

A 404 page, showing the requested URL:

A 404 page on a child theme, showing custom text and HTML tags in use:

A hierarchical taxonomy term, showing the content-not-found partial used on 404 and empty search pages in the river, because the term has fewer than 5 posts.

Why

For #692 and WE-103.

This is a hotfix because of the rapidity with which it is desired.

[benlk]

This is separate from and complementary to #1167, which may be updated to work with this code at any point.

[benlk]

Updated to display the Recent Posts widget with empty args:

[aschweigert]

Sorry, should have looked more closely at this. I'm not necessarily opposed to using wp_kses here (although I think it's overkill) but if we do it here...why aren't we using it for every text field in the theme options? For category/term descriptions? Everywhere else? It would make a lot more sense to have a globally allowed list of tags for user-entered description text. It's weird to make this the only exception and then it also makes the docs here way too heavy-handed. This should be re-thought.

[benlk]

Created new ticket for that question: #1172