refactored mainly styles and some identifiers

• removed all preview related styles in the plugin, since they are provided by the base
  since base commit 1f0be60 (in base branch “preview_styles” if not merged yet)
• replaced some “more_link” by "read_more" and removed all more_link-related styles, because they are useless and confusing
• replaced link identifiers read_more by what they actually do: link_to_detail

migration note: for updates
check all site templates and code for identifier
• replace “more_link” by “read_more”
• replace identifiers, that are replacing the href="{{read_more}}" by href="{{link_to_detail}}"
add new entry button string
add attr rel to link

• remove comment
Merge branch 'preview_styles'