Fix layout autodetection vis-a-vis Shoreditch
Fix layout autodetection vis-a-vis Shoreditch

Background
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The setting `mosaico_layout` defaults to `auto`. In this mode, it looks for Shoreditch and then
loads appropriate HTML/CSS (depending on whether it's installed).

Before
------

There's a subtle bug. Example situation:

1. Install Shoreditch
2. Neglect to set Shoreditch as the active theme
3. Navigate to "New Mailing"
4. The layout looks messy.

The problem is that the auto-detection rule is checking whether the
Shoreditch codebase *exists* -- not whether the Shoreditch look-and-feel is
being used on the current page-view.

The example above arises from misconfiguration.  A similar bug would also
arise if you had another customization which used hooks to toggle the active theme.

After
-----

The autodetection cares about whether Shoreditch is being used on this page-view.

Comments
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The general arrangement is still overly coupled and could be improved.  But
this at least makes it behave better and is minimal/safe/low-QA-work.

veda-consulting/uk.co.vedaconsulting.mosaico,veda-consulting/uk.co.vedaconsulting.mosaico,veda-consulting/uk.co.vedaconsulting.mosaico,veda-consulting/uk.co.vedaconsulting.mosaico