Make confirm dialogs themeable and consistent

Issue Summary

When costumising the style of confirm dialogs I noticed that the flag_content module behaved slightly different visually. There are three problems I encountered:

• The content type in the title isn't always ready for real use (eg content types of "user_comment" instead of just "comment"), so it looks strange for the end-user who has no idea what a content type stands for.
• The whole confirmation message is put in the confirm dialog's title, together with HTML formatting.
• There's no easy way (without patching/hacking) to change the way how all this is being displayed.

So I rolled out a patch of the changes I made:

• I've added two theme functions to the module, one for themeing the confirm dialog's description and one for the title.
• The title doesn't contain the content type anymore, no paragraph tags and the subject that is being flagged is between em tags (conforming with the way Drupal does it).
• I moved the descriptive part (A site administrator will review...) to the confirm dialog's subscription.

I'm not sure if this is a good way to do it, with those two themeing functions. I know these things can be changed by editing a .po file, but since these are meant for i18n I thought making it themeable was a better solution.