templates: refactor. fix usage of _layout and nested .ui-container

Squashed commit of the following:

- templates: media_resources: split _list from _wrapper
- remove obsolete templates
- sass: remove stray pixel line below .ui-tabs
- sass: fix nested .rounded .ui-containers
- sass: better css fix for nested .ui-container.rounded > .ui-container.bright etc.
- sets/entries: fix nesting of .ui-containers; consistent layout templating
- container nesting: fix /filter_set
- container nesting: fix /contexts
- container nesting: fix /entries/edit_multiple
- container nesting: fix /my
- container nesting: fix /explore
- container nesting: fix /custom_urls
- container nesting: fix /import
- container nesting: fix /media_resources
- container nesting: fix /media_entries
- container nesting: fix /permissions
- container nesting: fix /responsibilities
- container nesting: fix /search
- container nesting: fix /media_resources/browse
- inline unnecessary partials