[FIX] web_editor: force custom style in ui on non-custom link with style

This reverts commit [1] so as to avoid updating the DOM in two separate
places of the code for link edition. Said commit intended to prevent the
removal of custom styles on a button on which styles were applied
(erroneously) without applying a custom button class. This fixes that
issue by adapting the UI to show the button is custom if there are
custom styles applied. This way, if any change is applied, when
modifying the DOM, the custom class will be applied as well. This is the
order in which such corrections are usually applied with the snippets
bar.

[1]: https://github.com/odoo/odoo/commit/eb4edac560227b46efdcfe958e3b93e0b88def64



closes odoo/odoo#108615

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>