new settings form, several tweaks and compatibility with My Little Forum 2.4.18(.1)

Fixes

• invalid element def in a few icons caused black instead defined colour gradient in Safari browser
• when one folded the threads in the thread lists of the main views, the link for expanding the threads had no icon

Changes

• reimplementing the changes in context of the MLF versions 2.4.17 and 2.4.18
• use box-sizing: border-box; for all block elements, makes several occurences of calc in the forum CSS unnecessary
• rebuilding the main settings form in the admin panel without a table, first attempt, further changes are very likely
• structure unification of the pagination link lists in several places