Remove trimming of gmail.com for user accounts

[wkurniawan07]

https://github.com/TEAMMATES/teammates/blob/f70ae535d0d0586d48530704227b67f169f49bfb/src/main/java/teammates/common/util/SanitizationHelper.java#L53-L55

https://github.com/TEAMMATES/teammates/blob/f70ae535d0d0586d48530704227b67f169f49bfb/src/main/java/teammates/ui/servlets/DevServerLoginServlet.java#L63

https://github.com/TEAMMATES/teammates/blob/f70ae535d0d0586d48530704227b67f169f49bfb/src/main/java/teammates/ui/servlets/OAuth2CallbackServlet.java#L85

Removing the trailing @gmail.com is a practice forced to us by the old and deprecated GAE Users API. We do not have any reason to do that anymore, so this practice needs to cease.

Need to also migrate entities who are scarred by this practice: accounts, students, instructors. Namely, if the googleId does not contain any @, it should be appended with @gmail.com.

[daongochieu2810]

@wkurniawan07 do you think this is a good first issue?

[wkurniawan07]

Not at all. This issue involves data migration. Even putting help wanted there is already pushing the limits.