DDBNEXT-27: implement filter by section and first letter.
DDBNEXT-27: implement filter by section and first letter.

  TODO:
  - show message `No institutions match your search criteria`, when
    - first letter is selected _and_
    - one of the sector filter is selected _and_
    - the result is empty.

Deutsche-Digitale-Bibliothek/ddb-next,Deutsche-Digitale-Bibliothek/ddb-next,Deutsche-Digitale-Bibliothek/ddb-next