Enhance the filter on the "Fix 404 pages" form, allow to exclude file extensions

[olafgrabienski]

Description of the need

The "Fix 404 pages" list at admin/config/urls/redirect/404 is often crowded by files that don't exist anymore, especially images but also outdated PDF links and so on. The number of file and image related entries makes it hard to spot actual web pages in the list.

Proposed solution

Add an option to the filter above the "Fix 404 pages" form which allows to exclude certain patterns, e.g. common file extensions.

Alternatives that have been considered

Also a feature request, or an alternative approach: Backdrop could replace the list by a view. This would allow to customize the filter.

Screenshot example of the current situation:

Draft of feature description for Press Release (1 paragraph at most)

Backdrop now includes the option to exclude files from the form to fix "Page not found" errors.