Update to Django 2.2

Upgrade site and modules to Django 2.2. Remove and replace obsolete
functionality with current equivalents. Update requirements to latest
versions where possible. Remove unused dependencies.

Switch score calculation to pure Python

Rip out the score expression madness, and calculate the score in Python,
since I hear that Python is better at math than SQL is.

Fix score calculation hack

I have no idea why we do this in the database as some magic after we
call save(). I also have no idea why MySQL is seeming to think we want
type BIGINT UNSIGNED at the end of the
((2 * '+2' + '+1') - ( '-1' + 2 * '-2')) but it does.

Setting it to 2.0 or float(2) doesn't get the ORM to get this right, but
we are going to Decimal and making the 2 multiplier be of type Decimal
manages to make the ORM pull it's shit together and use something that
seems like we're okay with.

+1, -2 = 1 / 2 = -0.5 Score == True
Looks like it works.

UPDATE `symposion_reviews_proposalresult` SET `score` = CASE WHEN `symposion_reviews_proposalresult`.`vote_count` = 0 THEN '0' ELSE ((((2 * `symposion_reviews_proposalresult`.`plus_two`) + `symposion_reviews_proposalresult`.`plus_one`) - (`symposion_reviews_proposalresult`.`minus_one` + (2 * `symposion_reviews_proposalresult`.`minus_two`))) / (`symposion_reviews_proposalresult`.`vote_count` * 1)) END WHERE `symposion_reviews_proposalresult`.`id` = 1

Taking one out of PyCon's (US) book

We're lock-step with symposion, and upstream is dead.
Vendor it.