Use common authentication screen

[deepakmkoshy]

Since sign-in and sign-up page is similar, we can use the same for both merchant and customer. The only thing to note is that, when the customer chooses merchant or customer, store that decision in state and use it while doing the onPressed logic for sign-in and sign-up.

[deepakmkoshy]

Update 1dc8ad4 : Now Google sign in works for both merchants and customer. Still needs refinement to achieve single authentication for both.

[deepakmkoshy]

Added this feature in f1c2a09

Authentication files are now common to both merchants and customers and added to a separate folder.