Add activity notification during sign in process and add form validations

Summary:
* Fixes T1019
* show spinner and disable sign in button while signing-in process is running
* form validations
  * require username and password to have user input
    * disable the sign in button until they have user input

Test Plan:
* sign in and check the described behavior of the form / sign in process
* run the specs

Reviewers: shadowhand

Reviewed By: shadowhand

Subscribers: rjmackay, aMoniker, seth

Maniphest Tasks: T1019

Differential Revision: https://phabricator.ushahidi.com/D534