Fix EZP-20851: Uniform search casing for better pgsql support

This is a "pure" version of https://github.com/ezsystems/ezpublish-kernel/pull/492 – based on just master.

Branch fixing the field-case-insensitivity issues with PostGreSQL (and Oracle) on top of patrickallaert:EZP-20851 – @docb22 asked me to take a look at this.

The values in the database are already lowercased during indexing using the transformation processor in ezpublish-legacy. Thus the same transformation is now applied during search.

Once we implement indexing in ezpublish-kernel for the legacy backend we MUST make sure to also lowercase the values in sort_key_string.