Migrate image descriptions from image_comment_temp

Migrate image descriptions from image_comment_temp

image_comment_temp was always intended to be temporary, until an
expensive schema change on Wikimedia Commons's image table could be
done. Now that that has been done, stop writing image_comment_temp and
add a migration script to copy existing data into img_description_id.

Ic8efeddc will remove the reads from image_comment_temp and drop the
image_comment_temp table.

Bug: T188132
Change-Id: Iab5f521577a415b2dc213b517ee8a0dca4fdd0aa