Before this commit, Odoo used checkboxes in three different ways:

- A simple <input type="checkbox"/>, mainly in the frontend. The style
  is browser dependant.

- Same as above but with the BS3, checkbox structure. The style is still
  the same, but the alignement is supposed to be better (which is not
  always the case).

- The Odoo official structure:
	```
	<div class="o_checkbox">
	    <input type="checkbox"/>
	    <span/>
	</div>
	```
  which allows to have a cross-browser checkbox style and correct
  alignements.

The goal after this commit is to only use the BS4 *custom* checkbox
structure to achieve the same goal as our official structure (and
remove that one):

```
<div class="custom-control custom-checkbox">
  <input type="checkbox" class="custom-control-input" id="customCheck1">
  <label class="custom-control-label" for="customCheck1">...</label>
</div>
```

/!\ Labels are now required (use a zero-width space if necessary)

This is a simple renaming without adaptation.