backend: Fixes for Bengali and Devanagari

* The Bengali O and AU matras were wrong.
* Nukta characters have their own unicode codes, but can also be composed from
  the base letter and a nukta. We need to convert both to the same Braille
  character.
* Add tests for composite characters to Bengali and Devanagari.
backend/mappings/te.py: Assign letter RRA, use unicode for Vocalic L and LL
backend/mappings/ta.py: Don't remove unmapped NNA and SHA from input