Removed `\thesis@authorEn` and `\thesis@advisorEn`.

Added `\thesis@head`, `tail`, `author@head` and `author@tail`.

The `\thesis@authorEn` and `\thesis@advisorEn` macros are
unnecessary and can be emulated via

  `{\def\thesis@lang{english}\thesis@author` or `\thesis@placeholder}`

for an arbitrary language.