Separated PlaceholderAdmin into a mixin class to be used together with admin.ModelAdmin
Fixed a timing problem which occurend sometimes
Replaced PlaceholderAdmin against PlaceholderAdminMixin together with admin.ModelAdmin
Replaced PlaceholderAdmin against PlaceholderAdminMixin in unit tests
Replaced PlaceholderAdmin against PlaceholderAdminMixin in docs
fixed typo
Renamed FrontendEditableAdmin to FrontendEditableAdminMixin for using consistent terminology
PEP8 compliance
testing deprected PlaceholderAdmin
Merge pull request #2740 from jrief/fix-issue-2709

Fix issue 2709