A few bug fixes and enhancements

[aesmail]

• Use left_join when building search query. This allows more consistent search results across preloads when they are nil.
• Add ability to provide a string, or :safe tuple containing HTML to be rendered on a resource's index page as a description above the table.
• The :admin_footer can be specified in app config as nil, or String.t(), or {:safe, String.t()}.
• include many_to_many fields in fields_to_be_removed/1 function.
• increase minimum elixir version to 1.11.
• add phoenix_view to deps.
• allow more flexibility for admin_logo.
• allow more flexibility for admin_logo_mini.
• remove the "- Kaffy" suffix from page title.
• add decoding for array JSON fields.
• add support for composite primary keys.
• add the ability to "bulk delete" resources from the index page.
• hide the save and delete buttons on the show page based on available actions.
• apply :readonly and :editable to datetime fields.
• add Kaffy.Utils.auto_detect_resources/0 which returns Kaffy's attempt at discovering the list of schema/admin modules.
• default to nil for belongs_to assoc fields.
• fix issue with datetime fields sometimes updating unexpectedly.