Session names must be unique

[srmagura]

Our Session model requires their names to be unique. I think it would be better if this uniqueness was enforced only for sessions inside the same organization.

E.g.

• Both Organization1 and Organization2 should be able to have sessions named "Fall 2016". (doing this is currently rejected by our model validation)
• But Organization1 is not allowed to have more than one session named "Fall 2016". (this is how things are now, and that's how it should be)

[EricHorton]

3799630

This one requires a database migration.