Fix datepicker positioning when inside a scrolling container

The datepicker popup was not properly positioned relative to its input field
but to the first parent container with non-static position.

This fix places both the input field and the datepicker into a container
div with relative position. This means the position is calculated
properly even if some parent container scrolls.
Merge branch 'hotfix/datepicker-positioning' of github.com:ceari/echolot