Add tests to cover AccountsLogic
Do TODO, changed return type to void for some functions
Add assumption in getCourseInstitute
Add TODO in StudentsLogic - StudentAttribute should not be created  if course specified in it does not exist.
Move Account Level tests from LogicTest to AccountsLogic test.
Remove some unused imports.
Merge branch 'master' into Issue1918

Resolved Conflicts:
	src/test/java/teammates/test/cases/logic/AccountsLogicTest.java
[1918] Clean up AccountsLogic

fixes #1918