[REF] *: review scss variables handling

Unlike LESS, SCSS variables are not lazy loaded. Our system has thus
to be updated. This commit creates new templates which are t-called
in assets bundles (to replace the old less_helpers template):

- web._assets_utils: regroups the mixins and functions which *can*
  (and so should) be available in every asset bundle

- web._assets_primary_variables: regroups the variables (or mixins
  used as variables) which *can* (and so should) be available in
  every asset bundle

- web._assets_secondary_variables: same as above but provides an
  environnement where all the 'primary' ones are accessible. This is
  for example useful to handle the community/enterprise split:

  // Community primary variables
  $o-pink-color: pink; // enterprise color
  $o-brand-primary: blue;

  // Enterprise primary variables
  $o-brand-primary: $o-pink-color;

  // Community secondary variables
  $o-my-darker-primary: darken($o-brand-primary, 5%);

  => If there was only one variable template, enterprise edition would
     have been able to define its primary color at the end but the
     darker primary would not have been updated. Using the "!default"
     system and putting enterprise definition above would not have
     solved the problem as the $o-pink-color would not have been
     accessible.

- web._assets_backend_helpers: regroups the variables, mixins and
  functions which *can* (and so should) be available in the backend
  asset bundle only. This is especially (only?) useful for bootstrap
  variables overriddes.

- web._assets_frontend_helpers: regroups the variables, mixins and
  functions which *can* (and so should) be available in the frontend
  asset bundle only. This is especially (only?) useful for bootstrap
  variables overriddes.

Note: bootstrap variables are not accessible in any of those anymore.
If you have variables that should depend on bootstrap, you have 3
solutions:

- Find another way: your variable is probably useless, use bootstrap
  variables directly or create a variable that will influence the
  value of bootstrap variables. E.g. instead of declaring:
  `$myvar: $bootstrapvar * 3`
  and using $myvar alone, declare:
  `$myvar: 3` and use `$myvar * $bootstrapvar` where needed.

- Declare a copy of the bootstrap variable and use that one. In that
  case, you should also force-set the real bootstrap one to be sure
  they match (this should be done in appropriate templates mentioned
  above). E.g.
  ```
  $o-boostrapvar: 5;
  ...
  $boostrapvar: $o-bootstrapvar;
  ```

- Set your variable to null and set it to your bootstrap expression
  in the file you will need it (where bootstrap variables are accessible)
  without forgetting to add the !default flag to allow overriddes.
  ```
  $myvar: null;
  ...
  $myvar: $bootstrapvar * 5 !default;
  ```

This commit also partly changes the variable names to follow the
convention:
$o-<app_id>-<name> where 'app_id' is the current's app name or a
meaningful unique identifier ("theme" for all themes for example, as
no multiple themes can be installed).