The page picker is very slow on large wikis

Steps to reproduce:

1. Create a million pages.
2. Open the page picker by pressing Ctrl+G and type "Sand"

Expected result:

Suggestions are displayed within a second.

Actual result:

Suggestions aren't displayed for quite some time.

This is because the page picker uses an expensive database query. Indexes won't help with that as the query uses queries like %INPUT% with several fields and complex conditions. The whole REST endpoint used for this search should probably be changed to use the Solr search index.