404s for Plugins should return search results

[gibrown]

When a user lands on a plugin that is non-existent (could have been removed, or it could have never existed) we should turn the slug in the url into a search and try and get the user headed in the right direction.

[gibrown]

Add 404 redirect to search

[gibrown]

This patch redirects 404s to search using what would be the "plugin slug" as the search term.

I also included a fix for the ​https://wordpress.org/plugins/tags/ which was redirecting to the "search" plugin (or a 404 when logged out) rather than running a search because the search term was empty.

Some examples I used for testing:

• ​https://wordpress.org/plugins/the-social/ (closed, 404)
• ​https://wordpress.org/plugins/social-media/ (not yet visible to public, 404)
• ​https://wordpress.org/plugins/social-media-data/ (404)
• ​https://wordpress.org/plugins/tags/
• ​https://wordpress.org/plugins/search/ (this one is tricky. with this patch it redirects to searching for "search" which seems like the right behavior for any existing links out in the world)

[Otto42]

Seems to work correctly for both admin and non-admin users.

[Otto42]

In 5461:

Plugin Directory: Redirect 404s to plugin search using the slug as the search term. Fixes #2755. Props @gibrown