**Problem**
Before this commit, custom snippets were saved including elements inserted by
interactions (marked with `data-skip-history-hack`). These elements are not
intended to be saved, and their presence in the custom snippet could disrupt
the normal functioning of an interaction (see `s_countdown` example below).
This happened because interactions were not stopped before saving a snippet.

**How to reproduce**
1. Save a custom snippet including `s_countdown`. E.g. insert `s_text_block`,
   then insert `s_countdown` in it, and finally save it as custom snippet.
2. Place the newly generated custom snippet on the page.
3. PROBLEM: the countdown rings are not centered anymore. Inspecting  the page,
   one can see that `s_countdown` includes 8 canvas, the first 4 being invisible.

**Solution**
This commit introduces two new resources: `on_will_save_snippet_handlers` and
`on_saved_snippet_handlers`, which are called by `saveSnippet` before and after
cloning a snippet for saving. `EditInteractionPlugin` installs two handlers which
stop the interactions before saving a custom snippet and restart the interactions
after saving it. This way: 1. snippets are saved correctly; 2. we have a system
that can be used in future to do other processing before and/or after the saving;
and 3. interactions are not directly touched by `saveSnippet`.

task-4367641