Add email_access_valdiated_at field to user table, populate it

and update it when users have to use their email to interact with
Notify service.

Initial population:
If user has email_auth, set last_validated_at to logged_in_at.
If user has sms_auth,  set it to created_at.

Then:
Update email_access_valdiated_at date when:
- user with email_auth logs in
- new user is created
- user resets password when logged out, meaning we send them an
email with a link they have to click to reset their password.

migrations/versions/0313_email_access_validated_at.py

@@ -0,0 +1,43 @@
+"""
+
+Revision ID: 0313_email_access_validated_at
+Revises: 0312_populate_returned_letters
+Create Date: 2020-01-28 18:03:22.237386
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+revision = '0313_email_access_validated_at'
+down_revision = '0312_populate_returned_letters'
+
+
+def upgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.add_column('users', sa.Column('email_access_validated_at', sa.DateTime(), nullable=True))
+    # if user has email_auth, set email_access_validated_at on last login, else set it at user created_at date.
+    op.execute("""
+        UPDATE
+            users
+        SET
+            email_access_validated_at = created_at
+        WHERE
+            auth_type = 'sms_auth'
+    """)
+    op.execute("""
+        UPDATE
+            users
+        SET
+            email_access_validated_at = logged_in_at
+        WHERE
+            auth_type = 'email_auth'
+    """)
+    op.alter_column('users', 'email_access_validated_at', nullable=False)
+    # ### end Alembic commands ###
+
+
+def downgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_column('users', 'email_access_validated_at')
+    # ### end Alembic commands ###