Make Timespan-modification more flexible

When updating a 'Timespan', permit the modification of its associated
'TimespanAttribute's in the same POST.

Refactor attribute updating and deletion from 'postAttribute' to their
own functions 'updateAttribute' and 'deleteAttribute', and reuse these
for modifying attributes in 'postTimespan'.

Signed-off-by: Alexander Berntsen <alexander@plaimi.net>
Signed-off-by: Stian Ellingsen <stian@plaimi.net>
Reviewed-by: Stian Ellingsen <stian@plaimi.net>