textutils: updates string-to-ascii functions
- Amends translate_to_ascii function in textutils to fall back to the strip_accents function used in search_engine.py. This fixes a previously failing test-case in textutils.
- Moves the definition of strip_accents from search_engine.py to textutils.py. Also moves corresponding tests.
- Changes the import statements in modules using the strip_accents function accordingly.