Implemented randomly allocating students to periods

[hirokiterashima]

This PR requires a database change:

alter table `wise_database`.`runs` add column `isRandomPeriodAssignment` bit(1) not null default 0 after `end_time`;

Please test the following:

As teacher

• can create a run with/without "randomly assign students to periods" option
• can edit a run with/without "randomly assign students to periods" option (use the "Edit Settings" drop-down option in the main home page run listing view)

As student

• adding unit that is configured to randomly assign students to periods should assign a period randomly upon entering the access code and NOT allow student to modify the period selection
• adding a unit that is NOT configured to randomly assign students to periods should allow the student to select a period upon entering the access code

Resolves #52