This merge adds the background per section feature.
If the section to display has a background image configured, the background
will be refreshed using that section background image.
If the section has no background, the survey background is used.
The background is refreshed only if the previous and the next sections are
different. The background is refreshed at the same time that the next question
is loaded.

To ease technical maintenance and to keep it simple, the background is always
faded out on back or if survey has conditional questions. This is to avoid to
load all the survey structure and to copy the get_next_question logic in the
frontend. When loading the next question, we check that the question that will
follow afterwards has the same background. If not, we force the background fade
out when user goes to the next question (refresh_background_on_next).
Otherwize, we only fade out the question and not the background to keep a
smooth transition between questions and/or sections.

Note: The next section to display depends on free text (section with
description) configuration and on conditional questions.
The next section to display can be the next question's section or directly
the next question itself (if the question is a section).

Task ID: 2225393

Issue:
------
The `create` method (models.py) doesn't scale correctly
(with huge number of values) **when compute store no-readonly
field(s) are in values** (at most one).
It is due to the method `protecting` (api.py) which has a
bad complexity in the `create` situation
(list of (field, records) pairs as argument):

O(r²) with `r` = number of record containing the protected field:
List of `len(r)` send to `protecting`,
loop of this list (`r` factor), loop on fields (constant factor),
create a new frozen set with the previous one (`r` factor).

Fix:
----
Decrease the complexity to O(r) by creating a map of set of ids by
protected field which allows avoiding recreating a new frozenset
for each record (update with tuple of one inside => O(1)).

Performance gain:
----------
For a very simple model with only one compute store no-readonly field,
and all `create` `values` contains this field.
```
+--------------+---------------+---------------+---------------+----------------+
|   Batch ->   |     1000      |     10000     |    30000      |     80000      |
+--------------+---------------+---------------+---------------+----------------+
| Before (sec) | 0.083 ± 0.006 | 1.433 ± 0.091 | 8.764 ± 0.609 | 94.428 ± 3.021 |
| After (sec)  | 0.069 ± 0.003 | 0.706 ± 0.006 | 2.188 ± 0.076 | 5.875 ± 0.104  |
+--------------+---------------+---------------+---------------+----------------+
```