[FIX] web_editor: convert color opacity to be compatible with PIL Image color

ValueError 'unknown colour specifier' occurs when we access
export_icon_to_png(). This error occurs when we change the colour of an
icon in the mailing template, because the value of alpha (opacity) in 'rgba'
is in the range of 0 to 1, but PIL Image support colour opacity range 0 to 255.

This commit converts the opacity value range (0 to 1) to a range (0 to 255)
compatible with the PIL image library when the colour specifier is 'rgba'.

sentry - 3933353285

closes odoo/odoo#116476

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