Warnings fixed

The project no longer has any warnings, fixed by adding
contentdescription attribute in imageview and removing any unused
imports/variables.
Error-checking text fields, fixed small bugs

When a user attempts to create a new member without specifying a
username, email, or password, the dialog will close and a Toast message
will appear notifying them that those fields needed to be entered. I
also moved the segment of code that associated a profile_picture to the
current user so that it is only run after a succesful attempt is made to
create a new member. This was a problem before because a user could
attempt to create a new member without all the fields, and the segment
of code would still run.
Valid email check, new member dialog change

Beyond checking to see all the fields in the new member dialog are
filled, the email address is now checked to make sure it is valid based
on a regular expression. Also a custom OnClickListener was defined for
the new member dialog so that the user is notified of any problems
without the dialog closing.