clean up the unused comment in get_tags() and count_source_by_status() in Topic model
clean up the unused TYPES=() in Source model
correct typo
update how to do query on db objects
change SIGN
notes on designing n-leg fees
use product weight to calculate the postage fee now.

Before: find_postage_fee() and find_1st_leg() fee are called by find_max_purchase()
Now: find_1st_leg() is called by find_max_purchase(). self.weight is used by calculate the 1st leg.
add default value to Topic.weight due to migration, and migrate it
remove the default value of Topic.weight
remove the default value of Topic.weight, and migrate it
reorganize the admin form of Topic after adding a field Topic.weight
update notes & todo after setup a field Topic.weight for postage fee
find_postage_fee() returns Decimal, rathen then float now
part2 on postage fee calculation is made: find_packing_fee()
debug find_postage_fee() on the weight <= 2000g: a missing self.
change comment on max_purchase price
update requirements.txt
refine the number in packing, ebay_commsion, additional_fee
replace "，" or "。" or "." with a english comma "," in tab field when saving a Source
remove unused code in replace_non_comma()