Provide custom.css view for TTW CSS customizations without cloning a theme.

[MrTango]

Since we will use Bootstrap 4 and SCSS instead of LESS, we should also have a simple way of changing colors aso without having to clone a full theme.

The custom.css view in plone.app.thing will allow this.
The user can define some custom CSS in the Theming control panel, this get's stored in the Plone registry and rendered by the custom.css view.
The view will be registered as a ressource like any other ressource in Plone, but must be loaded at the very end to allow overriding other CSS.

part of #2967

[mauritsvanrees]

Status update:

• This was added in plone.app.layout master a while ago.
• Related PR Add custom.css resource after diazo bundle #3089 was merged to master.
• Today I have made Add custom.css resource after diazo bundle [5.2] #3117 to merge it to 5.2.

One thing I notice: the custom.css is also loaded on the Theming control panel. So Diazo is never applied there, but this new custom css is.
For me, this is fine. It is just not what I expected.

I guess theoretically this means you can lock yourself out. Quite simple actually: body {display: none} is enough. :-)
Anyway, there are more ways to shoot yourself in the foot with Plone. I am not worried.