LoD: Assign unique values to incoming LoD

refs #7070

Assign values starting at 1 to incoming LoD, since the promote/demote
code that swaps values has no effect when sortorder is all the same.
LoD: Cleanup fetch & display of levels of description

refs #7070

Fix get_atom_levels_of_description to work correctly with clear=False.
New LoD get unique sortorders, starting at the max value of the
existing sortorders, since swapping sortorder values will not work if
they have the same value.

Fix hiding of promote/demote arrows only when sortorder is the max or
min value (instead of 1 or length, since values may be deleted). Align
all demote arrows vertically. Use STATIC_URL instead of hardcoded value.

Clean up _atom_levels_of_description_sort_adjust remove unnecessary
iterate and inefficient swap.  Misc other minor fixes.