[FIX] website: avoid re-encoding the font family parameter

Since [1] the font family parameter was encoded twice when adding a new
google font. This causes an issue for fonts that contain spaces in their
name.

This commit removes that additional encoding and highlights the fact
that it already is an encoded parameter.

Steps to reproduce:
- Go to the Options tab while editing a website page.
- Open the Font Family dropdown.
- Select "Add a Google Font".
- In the "Google Font address" enter
`https://fonts.google.com/specimen/Open+Sans`.
- Click on "Save & Reload".

=> Field was highlighted as being wrong.

[1]: https://github.com/odoo/odoo/commit/d8cff9a37ba51c64628085b48d071ecb24e70f07

task-3256509

Part-of: odoo/odoo#117448