TablePager: Load images via CSS backgrounds rather than HTML <img>s

TablePager: Load images via CSS backgrounds rather than HTML <img>s

Moved files from directory: skins/common/images → resources/src/mediawiki/images
and renamed:

* arrow_disabled_last_25.png  → pager-arrow-disabled-fastforward-ltr.png
* arrow_disabled_first_25.png → pager-arrow-disabled-fastforward-rtl.png
* arrow_disabled_right_25.png → pager-arrow-disabled-forward-ltr.png
* arrow_disabled_left_25.png  → pager-arrow-disabled-forward-rtl.png
* arrow_last_25.png           → pager-arrow-fastforward-ltr.png
* arrow_first_25.png          → pager-arrow-fastforward-rtl.png
* arrow_right_25.png          → pager-arrow-forward-ltr.png
* arrow_left_25.png           → pager-arrow-forward-rtl.png

The new names are not very intuitive, but there's a mostly reasonable
system behind them and we need names like this to have them
automatically flipped for RTL styles.

Bug: 69277
Change-Id: Ica34cdd5fcc9340a94fb5e60bb34c30266953dcb