Accelerating some js calls, and changing the check-point for another.

[georgestephanis]

First change in the patch falls back from using jQuery to pure JS -- should be much quicker as it'll be loading frequently.

Second change lets us base it off of how the back-end calculates mobile, rather than just 'mobile' in the UA string. Should converge them to one test point, rather than dispersed ones.

Instead of dropping down to raw JS, why not cache it in a variable at the beginning? It's not like it will change.

I didn't want to add another global variable to check.

1. It doesn't need to be global; just move it up one level in scope
2. It should be faster than executing a .match() every time

jQuery seems to cache the selectors anyways, not sure if this would make any difference. On the other hand it's a good practice to set things like this to a variable instead of running it every time.

BTW isn't this a duplicate of #20557?

In [20677]:

Move the check for body.mobile in postbox.js to a variable as it's used in more than one place, props georgestephanis, scribu, fixes #20561