Rework timestamp labels

There were multiple bugs

• the default date format was wrong (dropped now cmopletely)
• most of the timestamps did a fallback to the date so most of the relative code was unused. We now never fall back but always use relative timestamps (as the original code intended).
• dropped several magic constants
• allows more flexibility for translators prompted by !708 (closed)
• fixes missing space after "Over " and "Almost " (which was never noticed due to the code (accidentally) being unused
• add tests

Fortunately this breaks fewer translatable strings as expected.