[IMP] website: introduce "auto" widget for theme modal

The theme customize dialog implements its own system of widgets. For
example: data-widget="color" and data-widget="input". Now, a new type of
widget data-widget="auto" exists. The goal is the same as the
data-widget="input" but with a predefined value that the user cannot
choose.

Also, as "data-value" was badly chosen during its implementation, it
is now renamed to "data-variable" and "data-value" is now used with the
new widget.

Example of the new widget "auto":

<opt data-widget="auto" data-variable="logo-height" data-value="3rem"
	string="Logo Height"/>

-> Allows to create a button forcing the logo height to 3rem.

Part of https://github.com/odoo/odoo/pull/33442
task-1974659