Added getters for the Add-Treatment form.
Moved filter models to the Experiment\Treatment\Filters directory.
fixed major ID validation.

IDs were incorrectly checked for their existence as values as opposed
to keys. The existence of major IDs are now correctly checked as
indices.
Fixed indentation.
Added error message for major_ids validation.
Renamed the session_tag_filter table to session_id_filter.
Deleted sessions relation.
Added dynamic rendering of treatments.