import/export terms table: remove use of old ids and use of n_words integer + make subforms declaration optional

[OK] Import dialog on terms page + merge function logging (TODO better CSS and possibility to import directly from other corpus

add "export terms table" option + corresponding logic and api route (ex: http://localhost:8000/api/ngramlists/export?corpus=4543)

groups in table: more detailed DELETE (subform params) and convert old POST method to PUT (for symmetry) (all params are in url)