Handle invalid URL error in image upload

The image url validation code throws an exception on invalid URLs which
we were not catching. This now catches it and displays an error message.

Fixes #558
Display new person page after saving if click save and add another

Uses a hidden field set by clicking the button to determine if it should
save and display the new person page or the page for the current person.

Fixes #560
make person political tab default tab

Fixes #555