admin can send join link to new instructor
1. redo the changes to the reverted project
2. now by default sample data will be imported for new instructor
3. modify the joinCourseForInstructor method to handle both instructors added by admin and other instructors
1.spilt joinCourseForInstructor into two methods
2.modify the verifyInstructorJoinCourseRequest's logic
3.fix typos
before creating multiple confirm methods
Refactored the method for checking the validity of the join request
added comments
minor layout refactoring
Merge pull request #2214 from TEAMMATES/72936492b

Suggested refactorings to complicated methods in AccountsLogic
add error msg for re-verification of new instructor account
Merge remote-tracking branch 'origin/master' into 72936492

Conflicts:
	src/main/java/teammates/logic/api/GateKeeper.java
	src/main/java/teammates/logic/core/AccountsLogic.java
	src/main/java/teammates/logic/core/InstructorsLogic.java
	src/test/java/teammates/test/cases/ui/browsertests/AdminHomePageUiTest.java
	src/test/resources/data/InstructorCourseJoinConfirmationPageUiTest.json
fix merge conflict
1.fix typo
2.add test case for admin adding instructor whose google id has already registered as a non-instructor account before
add necessary code to simulate the test case: logged in user click the joiningURL, his Google ID matches a non-instructor account and this account is associated with a instructor entity that has registered with the same GoogleID. After clicking the URL his account status will be updated to "isInstructor = true" and will be redirected to the InstructorHomePage with demo course.
1.delete unreachable branch and corresponding test cases
2.fix some typo
Merge branch 'master' into 72936492
[Fixes #72936492] revised: admin can send join link to new instructors