WIP: Add constraints

Adds constraints at the database level, and the changeset level.

Changes include:

• creating a unique index on plant names (database level)
• adding unique constraint on plant names (changeset level)
• adding a plant association constraint on disease (changeset level)
• adding a journal association constraint on entry (changeset level)
• adding user and plant association constraints on journal (changeset level)
• adding a plant association constraint on pest (changeset level)
• adding a plant association constraint on variety (changeset level)
• adding disease, pests, varieties constrains on the plant in plant_controller.ex on the delete function