Use $.data() less

[eritbh]

$.data() tries to read from jQuery's data cache or from data-* attributes. When it does the latter, it tries to process attribute values as non-string values if it thinks that's appropriate. It's rarely right.

Fixes #422 - using $.data('subreddit') on elements with a data-subreddit value with a subreddit name that's numeric was doing the wrong thing. $.attr('data-subreddit') always returns a string (and skips the jQuery data cache, which is also good for clarity.)

We should try to make an effort to remove other uses of $.data() as well, but this is all I'm gonna do for this PR.

[creesch]

LGTM, I honestly thought we already made an effort once to remove usage of it a few years ago exactly because of this sort of stuff...