DB: Add img_description_id column

DB: Add img_description_id column

This begins the process of merging image_comment_temp into the image
table by adding the needed column. Iab5f5215 will adjust the code
to use it and to add the necessary migration script.

Note this patch puts the new schema change in the 1.30 section rather
than the 1.31 section. This allows Iab5f5215 to have migrateComments.php
migrate the comments directly to the new field instead of having to
populate and then depopulate the temporary table.

Bug: T188132
Change-Id: I2485c5a758bf03bb2b4991eea920abd9d0d30bda