admin: apply LOWER() on journal filtering term for suffix/infix matching

Before this revision, LOWER() (via func.lower() of sqlalchemy) isn't
applied on journal filtering term for suffix and infix matching, even
though it is applied for prefix or immediate value matching.

This causes unexpected result with DBMS, which compares strings case-
sensitively by default.

This revision applies LOWER() on journal filtering term for
suffix/infix matching, to filter journal case-insensitively.