Error pages

Flash errors appear when needed for User signup and User login.

ALSO changed the wording on the Update Users button to say Add to pantry (and changed the test since it relied on that wording).

ALSO changed flash styling so that alerts show up green, notices are yellow and errors are red.

ALSO deleted 2 pointless files that didn't do anything (sessions/new and sessions/index). 

All 41 tests pass!