Refactor accepted_ct function on all tiles

Tests updated
Prefer single quotes
Refactor tests

Use assertIn and assertNotIn where possible.
Merge branch 'master' into issue_248

Conflicts:
	src/collective/cover/tests/test_embed_tile.py
	src/collective/cover/tests/test_file_tile.py
	src/collective/cover/tests/test_list_tile.py
	src/collective/cover/tests/test_richtext_tile.py