Course description: Allow reordering of sections

[ywarnier]

Is your feature request related to a problem? Please describe.
The order of the sections in the course description tool is too restrictive. When adding a new section later but wanting to put it in the first place, one has to edit each block to "shift" them one step below in order to add the new section at the beginning.

Describe the solution you'd like
Add a "display_order" field to the c_course_description table to enable sorting.
Add arrows up/down or a drag & drop option to enable reordering.