[FIX] web_editor, *: review icon buttons sizing and paddings

*: mass_mailing, website, website_sale

Before this commit, it was impossible to add `fa-fw` directly on a
`we-button` as it would make the icon half invisible, since the whole
`we-button` would have the font-awesome `.fa-fw` css rule applied on it;
width: ~1.28em.

Then, one would need to insert a `<i/>` tag inside the `we-button` for
it to work.

This commit improves that behavior by handling automatically such cases,
and will preserve the purpose of the `fa-fw` class which is to have a
fixed width for those icons.

Note that the util class introduced in [1] (o_we_button_icon renamed
here in o_we_icon_button) is still useful to handle the case of svg
images, they are not allowed to shrink, the same way the fa buttons are.

[1]: https://github.com/odoo/odoo/commit/e4233643b93b521454cc146c6a2ae876781fb9ba



Related to task-2172311

closes odoo/odoo#77980

X-original-commit: e8ab9d76
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>
Co-authored-by: Younn Olivier <yol@odoo.com>
Co-authored-by: qsm-odoo <qsm@odoo.com>