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. 👍

[tompave]

Thank you opening the issue.

Since in the meantime you've opened tompave/fun_with_flags_ui#43, and in fact this is a presentation issue with the UI repo rather than a logic issue with this core package repo, I'm going to respond there in the PR and close the current issue to keep things tidy and avoid duplication.