Avoid duplicating crowdanki_uuid when cloning note model

Fix #121, #123.

Unfortunately, this does not prevent a user from duplicating a note
model's crowdanki_uuid when cloning a note model on AnkiDroid or
AnkiMobile.  Hopefully most users create new note models on desktop!

crowd_anki/anki/overrides/__init__.py

@@ -6,3 +6,4 @@
 from . import cards
 from . import decks
 from . import exporting
+from . import models

crowd_anki/anki/overrides/models.py

@@ -0,0 +1,37 @@
+import anki.hooks
+from anki.models import ModelManager
+
+from ...utils.constants import UUID_FIELD_NAME
+
+def model_copy(*args, **kwargs):
+    """Override the built-in models.copy.
+
+    Extracting `_old` from `kwargs` is ugly, but allows passing
+    arbitrary arguments to the built-in copy function (`_old()`).  For
+    instance, in Anki 2.1.45, an optional `add` argument is added to
+    `copy`.
+
+    Note that this wrapper won't have any effect if `add=True` is
+    passed to `copy`!  Fortunately, this will only happen if another
+    add-on tries to clone a note model using `models.copy()`.  In Anki
+    itself, only add=False is passed, by design — the idea is that
+    normally the new model is only actually added (once!) to the
+    database in `Models.onAdd` in `qt/aqt/models.py`.
+
+    We could try to special-case `add=True` and call something like
+    `models.add(cloned)` if `add=True`, but it'd make this wrapper
+    more fragile to changes in `copy`, for relatively little gain.
+
+    A more serious problem than another add-on cloning note models is
+    AnkiDroid and AnkiMobile doing so, which we're currently not
+    mitigating.
+
+    """
+    _old = kwargs['_old']
+    del kwargs['_old']
+    cloned = _old(*args, **kwargs)
+    if UUID_FIELD_NAME in cloned:
+        del cloned[UUID_FIELD_NAME]
+    return cloned
+
+ModelManager.copy = anki.hooks.wrap(old=ModelManager.copy, new=model_copy, pos='around')