added custom template field to hostelement and element on backend
fixes, migration
added support for the custom template flag in the editor. Disabled changing of the template for prepared and running elements.
changed custom_flag db entry from column to attribute
Merge branch 'master' into custom_template_flag

Conflicts:
	backend/tomato/elements/generic.py
Merge branch 'master' of ssh://github.com/GLab/ToMaTo into custom_template_flag
Merge pull request #343 from t-gerhard/custom_template_flag

Custom template flag