[REF] website_blog: refactore cover option and change opacity behavior

* Isolate website_blog style
* Simplify/Correct the cover option JS
* Allow to modify the cover from the blog list page
* Change cover layout (use a more conventionnal odoo design with a
div which is full width, which contains a container(-fluid) then its
content) -> easier to customize.
* Refactore code to be understandable
* Change behavior of opacity filter: before this commit the cover
filter was not intuitive and did not work on IE or Edge, it mixed
the color of the bg-image and the bg-color in a complex way and the
opacity was just controlling if the image was visible clearly or not
(which led in general to ugly style if the opacity was used). Now, the
color in applied *on top* of the image so that its color changes the
hue of the underlying image. The opacity value controls only the
opacity value of the color to mix.

[MIGRATION] Note: as the opacity behavior has changed, the exact same
behavior is not reproductible at all. But for the change to make a bit
of sense in old databases, this should be applied for the field
"cover_properties" of blog posts (with changes in layout):

if cover_properties.background-color != "oe_none":
    cover_properties.opacity = 0.6
else:
    cover_properties.opacity = 1.0 - cover_properties.opacity

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