Update noteable after a new note is added

**What does this do?**
It makes sure that whenever a new note is added to an noteable item, the
updated_at of that item is also updated.

**Why is this needed?**
At this moment when you post a comment on an issue or add a label to an issue,
the updated_at is not changed. Because of this the filtering for least recently
updated is not really useful (since it only takes in account the original text
from the noteable).

Signed-off-by: Jeroen van Baarsen <jeroenvanbaarsen@gmail.com>