DB: converted VARCHAR2 to NVARCHAR2

- On Oracle use NVARCHAR2 instead of VARCHAR2, since it ensures data
  are in UTF-8 format + column limit semantic means characters, not bytes.
  Limit of 4000 bytes on data in column is still present.
- Updated DB version to 3.1.15
- Added insert with current DB version to end of file, since it's value is
  expected to be present even in empty DB.
- Removed some whitespace from inside SQL statements.