Significantly speed up the process by copying Gallery2 thumbnails and
resizes wherever possible instead of regenerating them. This requires
us to figure out the dimensions of the original G2 derivative and make
sure that it matches in some reasonable way.
To allow users to take advantage of this, calculate the optimal thumb
and resize size to set in G3 to match what was used in G2. While
we're at it, give the user some idea of how much data is available in
G2 to import.
Notes:
* Don't store the g2->g3 map in the task; it gets too large and
overflows the context column.
* Import album sort orders
* Try to treat duplication in the summary and description fields
sensibly.
* Move the g2 import config page under the Settings menu
* Clean up the settings page slightly and send users to the
maintenance page to do the actual import (not an optimal UI, but it
works).
