Backend - application_number, registration_number & username `ilike` pattern

Follow up to our earlier discussion:
Currently `application_number`, `registration_number` & `username` implement regular B-tree index and the `ilike` operations on these are `%term%` patterns.

With this pattern and index type, it would often do full table scans.
We can update the pattern to `term%` which will be very fast. But this would mean, let's say to filter on username in Test to enter `idiir\...`.