Confirm before changing a source's journalist designation

Fixes #550. However, it displays the confirmation every time, which makes it annoying to use multiple times in a row (for example, if you were repeatedly cycling through random codenames until you got one that you liked).

The obvious solution here would be to add a checkbox to the dialog, something like "Don't ask me again" or "Always confirm before changing source codename". However, that would require creating a custom dialog with JS (there is no way to add a checkbox with Javascript's built-in `confirm` function). You would also want to fully support it by providing a prefs page for the journalist to change their selection in the future.

Given all of this, I'm inclined to target this for 0.4 or 0.3.x, and not 0.3. I don't think it's a major problem that we don't ask for confirmation before changing a source's journalist designation since it doesn't contain any useful information.

@runasand, @micahflee I would appreciate your input on this!