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.
Resolve errors, and recommit the changes.

See:
https://github.com/Automattic/jetpack/commit/380dd99ef8a761f42f00a2c03b5
c09636279441c
Upload pre-commit hook to repository.
Keep pre-commit hooks in sync.

Or at least notify folks if theirs is out of sync.
Declare module a global.
Open Graph: restore the jetpack_open_graph_image_default filter

It was added in c1c6123ebb8fcbe3f0e52df097b4c2ed5a874b1d
then removed in 7738ace0b6202597fd01bda9dd3020201d64aad7
Minor CSS tweaks & tagline change

- Left aligned JP homepage toolbar navigation & logo
- Change tagline
- Fixed search icon formatting on homepage
Merge pull request #359 from Automattic/ui-tweaks

Minor CSS tweaks & tagline change
Ability to filter modules by category

- Added filter by category functionality to JP homepage
- Removed “popular” filter from JP homepage
Merge pull request #360 from Automattic/ui-tweaks

Ability to filter modules by category
Random redirect: No need to check for conflicts on wpcom.
Custom CSS: Remove code that should not be in Jetpack

Merges r93900-wpcom.
Correct version of 6df037d

The function `_x` only takes three parameters.
SSO: Fix issues with WP logo on button.

Previously it didn’t account for core’s button resizing breakpoint.  By
changing it to a flexible height, it will fit anything going forward.
Revert "Publicize: Fix the ability to mark posts as not publicizable with wpas_submit_post? filter."

This reverts commit 2beb14cce962d53a05df0f655f5db47d5fdbaad3.

This changeset was leading to issues where users couldn't publicize posts that were done from any UI other than the web interface.
Updated module descriptions & icons added to JP homepage

- Updated module descriptions
- icons on homepage
- Removed dummy “config” link from homepage “learn more” modal
Merge pull request #368 from Automattic/ui-tweaks

Updated module descriptions & icons added to JP homepage
Add icons to modules page

Icons added in rows on modules page.
Merge pull request #369 from Automattic/ui-tweaks

Add icons to modules page
Learn more modal copy, and check all checkbox alignment

- Select all radio alignment on modules page
- Learn more modal copy showing up on homepage