With less2sass and bs3tobs4 tasks, the assets structure was reviewed to
handle the specificities of both framework and to prepare for our next
odoo tasks. In particular, the assets_helpers and bootstrap overrides
were basically split into 4 parts: utils, primary variables, secondary
variables and bootstrap variables.
(see https://github.com/odoo/odoo/commit/6e4db7d13a926845bf82f5b035afede42d6d5a89)

Using the !default system for the bootstrap variables parts seems now
a good improvement. This is part of what this commit does: adding the
default flag for all bootstrap variables overrides and inverting the
order of the files in the _assets_backend_helpers and
_assets_frontend_helpers templates. This allows to avoid code like this:

portal:
```
$body-bg: white;
```

website:
```
@if $var != null {
    $body-bg: $var;
}
```

This code makes the body white with portal or equal to $var with website
if $var has a value. The same behavior in the new file order is achieved
with:

website:
```
$body-bg: $var !default;
```

portal:
```
$body-bg: white !default;
```

This order is now also followed for the _assets_secondary_variables
templates (there are only a few).

This commit also make better assets hierarchy by making website inherit
from portal assets (instead of web) and portal inherit from web_editor
assets (instead of web) (thus relying on strong dependencies instead of
installation order which is not always right for migrated databases).