Infinite: Only load new pages when within two screens of the bottom.

The current infinite scroll code loads a new page when the browser is
within 25% of the bottom of the page. On very long IS pages, this leads
to loading an increasing number of unnecessary pages, as the bottom 25%
of the page increases in size as the user loads more and more pages.

This patch makes IS less aggressive in loading new pages. Two screens was
chosen as a starting point and is obviously open to tweaking.
Redirect folks to the configure page.

This is temporary until it can be retooled to display the form in the
modal.
Merge pull request #380 from Automattic/ui-tweaks-dave

Playing with alert message location & styles
Pull conflicting plugin lists to the top of the class.

Easier to maintain up here without having to eternally search for them.
Consolidate how we check for active plugins.

Instead of doing it the same way three places, use a function!
Braces are good.
Use the previous declared variable name.
Merge pull request #299 from blowery/infinite-load-criteria

Infinite Scroll: Improve detection that next page should be loaded.
Tagline and Messages formatting

- Fixed tagline width when logged in
- round 3 of messages formatting