Tweak Mobile Viewport Layout

CHANGELOG
General:
- Added additional media query rules for responsive post thumbnail
images if CSS background is used.
- Created new page posts-photos-only.html.
- HTML markup cleanup.

Posts Page:
- Fixed double border issue on mobile viewport.
- Fixed padding issue on tablet viewport.
- Added responsive posts page info (Page x of xxx & xxx Total Items) on
top and bottom of the page.
- Hide other thumbnails in medium and small viewport when in 3 column
grid layout to preserve layout consistency across all viewport
breakpoints.
- Added responsive pagination.
- Hide filter buttons (all posts, photo only, etc..) on mobile
viewports.
- Rename posts.html to posts-all-posts.html for organization purposes -
since the posts page have 4 sections.
- Rename _postpage.scss to _postspage.scss - to not confuse to the
actual “post” page of each post.

Home Page:
- Fixed double border issue on mobile viewport.
- Fixed padding issue on table viewport.
- Hide filter buttons (latest, popular, japan, etc…) on mobile
viewports.