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.

[scribu]

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

[georgestephanis]

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

[scribu]

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

[azaozz]

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?

[azaozz]

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