Refactor the general flow of g2_import_task::import() to simplify it
and make it more sensible. It had gotten a little overgrown. It's
still a little overgrown, but I don't see an easy way to simplify it
from here.
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).
around in the task so that on subsequent import runs we don't try to
reimport stuff we've already pulled in. This also gives us a mapping
so that we will be able to translate old G2 urls into the hot new G3
urls.
