As an editor I to manage the display of issue covers on all themes

[joemull]

User story

As an editor I may want to display issue covers at myjournal.org/issues/, or I may want to hide them, on any theme.

Currently the clean theme hides them and there is no setting to show them:

Proposed solution

• [ ] Decide how issue covers should appear on the clean theme, maybe with a smidgen of design feedback
• [ ] Decide how a lack of issue covers should be displayed on OLH and material themes
• [ ] Make a setting that controls whether issue covers display on /issues/, with a default of 'on'
• [ ] Refactor the templates for all themes so that the /issues/ page follows the setting
• [ ] Add a migration for current clean theme journals to set the setting to 'off'
• [ ] If feasible, add this setting to any startup wizard or checklist so that new journal editors are presented with it as configuration decision they should make