Clean up miniview and search templates

[vsalvino]

• Add miniview and search template for Article and Event pages.
• Use miniview for Article Index and Event Index pages, for consistency.
• Make miniviews renderings more consistent throughout the site.
• Uniform height of cards when rendering miniviews in a row, e.g. related pages or latest pages.
• Fix heading levels in miniviews and search templates to fix accessibility warnings.
• Miniviews now have two new variables to control heading size and card styling, which can be passed in: {% include .. with h="h3" and miniview_css_class="my-class-here" %}

Fixes #578, #647, and #660