[FIX] web_editor: prevent crash when deleting multiple snippets in one deletion

Since saas-12, when deleting contents, parents are deleted too if they
become empty (while it was not possible to delete contents in that case
before). The problem here was that the history was recorded multiple
times if one deletion made multiple snippets be deleted and this
sometimes could crash.

Now the history is recorded only on first deletion when the user ask
for it.