Adding secure tag to cookie

[Luanmq]

Fixed security hotspot related to creating a cookie without the secure flag. To learn more about this issue visit: OWASP

[github-actions]

Hi @Luanmq, thank you for your interest in contributing to TEAMMATES!
However, your PR does not appear to follow our contribution guidelines:

• Title must start with the issue number the PR is fixing in square brackets, e.g. [#<issue-number>]
• Description must reference the issue number the PR is fixing, e.g. Fixes #<issue-number> (or Part of #<issue-number> if the PR does not address the issue fully)

Please address the above before we proceed to review your PR.

[wkurniawan07]

teammates/src/main/java/teammates/ui/webapi/JsonResult.java

Line 58 in 6af7a27

cookie.setSecure(!Config.IS_DEV_SERVER);

Secure cookie flag has been set in here.

Closing as the change is redundant.