* website, website_sale, website_sale_delivery

While we used BS3, we used our own responsive classes for text
alignment (text-right-sm, text-center-not-lg, ...). Now BS4 classes can
be used to achieve the same behavior so there is no point in declaring
our own ones anymore.

Also, as the breakpoint names changed (lg -> xl, md -> lg, ...), the
current code was not working as intended anymore anyway.

* website_sale

Bootstrap chose not to implement this feature, probably relying on their
grid system (see https://github.com/twbs/bootstrap/issues/21943).

We have a case where a responsive size of 0px is needed though: applied
on a table, this forces the table to its contents' natural width. So
this commit implements the behavior and adds the size of 0 to replace
the non-responsive class 'o_table_column_fit'.

media queries that target a specific screen size handled by bootstrap
have now a more convenient way to be defined:

```
@​media (min-width: $screen-sm-min) { ... }
->
@​include media-breakpoint-up(sm) { ... }
```

```
@​media (max-width: $screen-sm-max) { ... }
->
@​include media-breakpoint-down(sm) { ... }
```

```
@​media (min-width: $screen-sm-min) and (max-width: $screen-sm-max) { ... }
->
@​include media-breakpoint-only(sm) { ... }
```

```
@​media (min-width: $screen-sm-min) and (max-width: $screen-md-max) { ... }
->
@​include media-breakpoint-between(sm, md) { ... }
```

Also, bootstrap has changed the values for its default screen sizes:

------------------
     | BS3 | BS4 |
------------------
0    | XS  | XS  | -> Portrait Mobile
576  | /   | SM  | -> Landscape Mobile
768  | SM  | MD  | -> Portrait Tablet
992  | MD  | LG  | -> Landscape Tablet / Small Desktop
1200 | LG  | XL  | -> Desktop
------------------

While it is possible to configure BS variables to behave as BS3, we took
the decision to keep the BS4 variables untouched. BS4 system seems
indeed better and having the purest bootstrap possible is better for
Odoo to be used as a framework.

Convert content so that the assets compile on app installation. The
style is still broken after this as the variables/mixins/... are not
defined in the right order (as it did not matter in LESS but does in
SCSS).

This commit basically changes:
- Variables: @​var_hello -> $var-hello
- Mixins: .mixin_world() {} -> @​mixin mixin-world {}
- Classes used as mixin: .my_class() -> @​extend .my_class
    - Here there were no other solution than to convert the use of
      a mixin call by the use of an extend as a first approximation
- LESS functions -> SCSS functions (e.g. fade -> rgba)
- Move first variable definition before the variable is used
    - Still need to make sure last variable definition is at the
      right place

This is a simple renaming without adaptation.