Some improvements with didactic value

[pimoGit]

AUTH:

• made a compressed version of form validation;
• fixed a bug about persistence of error messages across pages;
• refactoring signUp and signIn actions, no repeated code anymore;
• refactoring/optimization signUp component, leaving also the original version commented out.

MIDDLEWARES

• added key to the list items;
• changed a callback function on props.map in user_list.js component to make it clearer.

TESTING:

• differentiated the state name from the prop mapped name on comment_list_test to make it clearer.

ALL:

• added some comments;
• got rid of Gmaps useless reference.

I think all these changes have a didactic value.