Make percentage input consistent

[s3cur3]

Hi there! Long-time happy FunWithFlags user here. ☺️

I recently started using the Percentage feature, and found the input somewhat error prone. For instance:

In the input field, I'd call a value between 0 and 1 a "ratio" or maybe "proportion" of users. Furthermore, the fact that previously-submitted values get displayed as an actual percentage ("5.0%") while new inputs must be a ratio makes it easy to accidentally scan the page and try to input, say, 10 to update the percentage.

To my mind, the right thing to do here is to accept input in the range (0, 100) while storing it in the database as the ratio [percentage] / 100. I'm happy to put together a PR if this is something you'd like addressed. 👍