Enforce eight new lints

• Enforce 8 new lint rules:

  • avoid_single_cascade_in_expression_statements
  • await_only_futures
  • prefer_inlined_adds
  • sort_child_properties_last
  • unnecessary_brace_in_string_interps
  • unnecessary_getters_setters
  • unsafe_html
  • use_full_hex_values_for_flutter_colors

• Mark a number of lints unused, see README.md for details.

• Remove avoid_empty_else as it is redundant when
  curly_braces_in_control_structures is also enabled.