Two new query methods to find empty/non-empty sektions
Added method to render .single-lines within columns

The number of columns in inferred from the # of div.column within
 the specified target
Defined behaviour of clicks on .single-lines that are within columns

For now, its the same as click on any other single line anywhere else
Sharpened logic for replacing any :prev or :id within data-action
[sektions]: Update action sends back its own JSON

JSON has info on which students were removed from a sektion and therefore
which names should now be hidden
[teacher]: Modified interface for editing a sektion
Merge branch 'dev'