Link Service to Send Invitation button; now sends out emails

[will-molloy]

Solution to #71

• Created a new REST endpoint POST /api/studies/send in StudyResource using new StudyService object
• Created a new method StudyService#sendInvitationEmail(Study)
• Linked POST endpoint to Send Invitation button via study-detail.component#sendInvitation() and study.service#send().

Currently using a test gmail account (tt7199425@gmail.com) as I don't know how to authenticate the users email (possibly related to #37). However, it is successfully sending invitations from this gmail account.

Test:

1. Login as admin (admin:admin).
2. Create participants and a study.
3. View the study and click Send Invitation.
4. Participants now receive invitation emails.

[pulkitkalra]

Firewall for some antiviruses (E.g. avast) was blocking the email from being sent. @wilmol This should fix it: props.put("mail.smtp.ssl.trust", "smtp.gmail.com"); In the StudyService, method: getUserEmailSession()

Not sure about the impact of this from a security point of view

[softeng-701]

@wilmol Please resolve the conflicts asap, thank you.

[will-molloy]

@softeng-701 fixed :)

[softeng-701]

@wilmol Awesome!

[AprajitGandhi]

Tested it, good unit tests - they all pass too. Good commenting. Approved