Add styles to the newer / older post pagination

[carolinan]

The newer / older post pagination is missing styles and does not yet match the initial screenshots.
Pagination.php should also be moved to the parts folder.

A decision needs to be made regarding #93 ,
Icon-only controls

In the mobile screenshot, only the arrow icons show and the text "Older / Newer posts" is hidden.

If we choose to show both the icon and text, there is not enough room on small screens
to show the text, arrows and page numbers in one line.

Instead, the navigation could be split into 3 lines:

And two lines if only one of the texts are displayed:

[carolinan]

Reopening because when you are on the first and last page, the numbers are not centered.