XWIKI-8807: Implement sorting in the Solr Search UI
- Removed the unused "<field>_exact" fields and kept only space_exact that is used in the Search UI
- Added the "lowercase" field type and defined title_sort and author_display_sort used for sorting in the Search UI
(cherry picked from commit f21aad42d45535c26cf6d957cc432c69b7240908)