Add custom.css resource after diazo bundle

[MrTango]

Insert virtual custom.css bundle after Diazo bundle.

[mauritsvanrees]

For me it is fine to include this in 5.2. It makes small site-specific styling changes easier, which is good.
Does the @plone/framework-team want to weigh in on this?

The upgrade step would need to be backported to 5.2 then.

[ale-rt]

I added a note in the "FWT Meeting Protocol 2020-05-05" shared document!

[mauritsvanrees]

I see the team meeting was supposed to be yesterday. Was this discussed?

(Needs an upgrade step in plone.app.upgrade for 5.2, like was added for 6.0 already.)

[ale-rt]

We rescheduled the meeting to next Tuesday (2020-05-12)!

[ale-rt]

Maybe it is better to add some condition to not always append the custom css?
It makes no sense if it is empty, right?

[ale-rt]

The @plone/framework-team approved the backport of this PR to 5.2 https://community.plone.org/t/fwt-meeting-minutes-2020-05-26/12344

[mauritsvanrees]

I am pretty sure this needs an upgrade step for <records interface="plone.app.theming.interfaces.IThemeSettings" /> to include the new field, like @MrTango did in https://github.com/plone/plone.app.upgrade/pull/230/files for Plone 6.0. But I don't see a problem on the theming control panel when switching from a version without this field to a version with this field. This surprises me.
Maybe I got confused when running into unrelated errors though.

@MrTango Could you check that? And could you backport this CMFPlone change from master to 5.2? (Otherwise I could do it.)

[mauritsvanrees]

I checked again. Started with a fresh Plone 5.2.1 site, copied its database to coredev, ran the Plone upgrade. The theming control panel looked fine, but when I save the Advanced tab, with no changes, I get this error:

Traceback (innermost last):
  Module ZPublisher.WSGIPublisher, line 162, in transaction_pubevents
  Module ZPublisher.WSGIPublisher, line 359, in publish_module
  Module ZPublisher.WSGIPublisher, line 262, in publish
  Module ZPublisher.mapply, line 85, in mapply
  Module ZPublisher.WSGIPublisher, line 63, in call_object
  Module plone.app.theming.browser.controlpanel, line 65, in __call__
  Module plone.app.theming.browser.controlpanel, line 215, in update
  Module plone.registry.recordsproxy, line 59, in __setattr__
AttributeError: custom_css_timestamp

[mauritsvanrees]

Ah, plone.app.theming has its own GS profile, and CMFPlone automatically applies its upgrade steps. So if we add an upgrade step to reload the settings, this fixes the problem. Done in plone/plone.app.theming#184 in this commit: plone/plone.app.theming@a360731.

Technically, the plone.app.upgrade changes for 6.0 could be reverted, but it's fine to leave them (and the part of those changes that creates a v60 structure is still needed anyway).

So that only leaves backporting the CMFPlone changes of the current PR to 5.2.

[mauritsvanrees]

I made a PR: #3117