Users home feeds now display only their tweets.

In addition:
Users pages can now only be accessed by rootURL/username instead of
rootURL/users/user_id
Took out the view tweet and tweet index pages.
Added helper methods inside of sessions_helper.rb to help verify the
current user logged in.
Tweets can only be edited by the user who created them.
Users cannot see the 'Edit' or 'Delete' buttons except for on their own
home page.