Force syncing of media after import

Otherwise, if in the source CrowdAnki decks a file has only been
edited, this won't be synced to other devices.

By default, Anki only checks whether it should more expensively check
for changes to individual media files if the mtime of the entire media
directory has changed.

(According to the docs:

https://docs.ankiweb.net/syncing.html#media

> It will notice when media has been added, removed or replaced in
> your media folder, but will not notice if you have made edits to
> existing files.

However, the mechanism by which it checks for this is via the
directory is (currently!) via the mtime of the dir:

https://github.com/ankitects/anki/blob/b7cb0c0d0081202586fd2d88541db962819736b3/rslib/src/sync/media/database/client/changetracker.rs#L94

)

Note that this means that every time CrowdAnki imports a deck, with
media, Anki will have to perform the more expensive check.

The cost of this "second" check was deemed sufficiently high that the
current behaviour (of two checks) was (re-)introduced here:

ankitects/anki@35cbde6

However, given that importing CrowdAnki decks is not a frequent
occurrence (probably less frequent than adding new media files oneself
— which also triggers the "second" check), this is a worthwhile
change.

For background see here:

anki-geo/ultimate-geography#638 (comment)

crowd_anki/importer/anki_importer.py

@@ -54,6 +54,11 @@ def import_media(self, directory_path):
                 full_filename = os.path.join(unicode_media_directory, filename)
                 if os.path.isfile(full_filename):
                     shutil.copy(full_filename, self.collection.media.dir())
+            # Set media dir mtime to force sync of media.  Otherwise,
+            # if no media files are added or removed, Anki will not
+            # sync media changes.  See the Anki source:
+            # https://github.com/ankitects/anki/blob/b7cb0c0d0081202586fd2d88541db962819736b3/rslib/src/sync/media/database/client/changetracker.rs#L59
+            os.utime(self.collection.media.dir())
         else:
             print("Warning: no media directory exists.")