Center nav on small devices.

[geospatialem]

This PR centers the navigation and dropdown menu on smaller devices (phones specifically).

The current look on small devices:

The proposed look on small devices:

The proposed change will:

• Add text-center to the nav navbar-nav class in the index.html
• Add center justification the dropdown menu in app.css.

Note: The proposed change affects the class in app.css as it changes from .dropdown-menu a i to .dropdown-menu a.