Changing CSS in a note model doesn't propagate (to other devices) after import

[aplaice]

When only the CSS is changed in a note model (in a deck.json), then after an import this change is not propagated to AnkiWeb and other devices.

To reproduce:

1. Import a CrowdAnki deck.
2. Sync to AnkiWeb.
3. In the deck.json for the deck, change the CSS (.note_models.[].css) for one of the note models.
4. Import the CrowdAnki deck again.
5. Sync to AnkiWeb again.
6. In another Anki instance (e.g. other profile/other computer) sync to AnkiWeb and check the CSS for that note model.

(Step 6 is needed because AFAICT AnkiWeb doesn't show the raw CSS. (I'm not 100% sure if the CSS is applied in AnkiWeb — if it is and the CSS change is clearly visible you can probably check visually (I had changed padding which didn't really show up).)

Expected behaviour:

The CSS change is applied locally (in Anki) and is synced to AnkiWeb/other devices.

Actual behaviour:

The CSS change is applied locally (in Anki), but is not synced elsewhere.

Diagnosis

https://github.com/Stvad/CrowdAnki/blob/master/crowd_anki/representation/note_model.py#L29

        field_names = ("flds", "tmpls")

should probably be:

        field_names = ("flds", "tmpls", "css")

NB This is unrelated to #236 — the bug is present both before and after applying #236.

(Changing, say the front/back templates does require a full sync and hence does propagate.)

Aside: it seems that Anki itself now doesn't require a full sync after changing templates and/or CSS (within Anki itself), and the changes do propagate as expected. Maybe we should copy what Anki does both for CSS and templates? (For fields a full sync likely still makes sense (?).)