Added share playlist functionality.

Visiting the root path and then clicking a link is a bit redundant when we can go directly to the link. Remember that this is a helper and not really a test: we have another test that verifies a user can sign in from the root path.

```ruby
visit "/users/sign_in"
fill_in "Email", with: user.email
fill_in "Password", with: user.password
click_button "Log in"
```

I would also take out the expectation at the end.