Cannot select Custom Header or Background when searching in the Media Library

[hd-J]

Here is a missing element of the new select from Library too introduced in #19840.

Steps to reproduce:

1. Go to Appearance > Header, or Appearance > Background, and click on "Choose image"
2. Instead of selecting an image from the ones you see, search for a term using the search form
   • The search result does not include a link to "set as background" or "set as header"

Also, hidden tabs ("From Computer" and "From URL") reappear after pressing "Search Media" button.

20856.patch​ preserves the context in media_upload_library_form().

There's also a suggestion to add a "context" parameter to get_upload_iframe_src() in ticket:19396:8 (#18708 and #20632 would benefit from that as well), we could probably do that in 3.5.

In [21008]:

Preserve context when searching in the media upload form. Props SergeyBiryukov. fixes #20856