[FIX] web_editor: fix url regex on Safari iOS

Commit [1] introduced a stronger regex to detect urls. Unfortunately,
it did so using a negative lookbehind, which is is only supported on
very recent version of Safari iOS.

This commit is an attempt at reproducing the same behavior, albeit
with a more complicated regex, unfortunately.

[1]: https://github.com/odoo/odoo/commit/a42a19e484d63cebeaed10b75ac20e507ff85d56



closes odoo/odoo#134821

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>