Update order status after fullfilled digital products

[korycins]

We don't update order status after the automatic fulfillment of digital products.
Update of order history was missing.

Related to #4682

[django-queries]

Here is the report for 693f41f (korycins/saleor @ fix/digital_product/update_order_status_after_fillfiled)
Base comparison is 03eb427.

No differences were found. (click me)

# api.benchmark checkout
  test name                           	left count 	right count	duplicate count
  ------------------------------------	-----------	-----------	---------------
  add billing address to checkout     	         34	         34	             20
  add shipping to checkout            	          7	          7	              0
  checkout payment charge             	         14	         14	              0
  complete checkout                   	          6	          6	              0
  create checkout                     	         48	         48	             24

# api.benchmark homepage
  test name                           	left count 	right count	duplicate count
  ------------------------------------	-----------	-----------	---------------
  retrieve main menu                  	          5	          5	              0
  retrieve product list               	          4	          4	              0
  retrieve secondary menu             	          5	          5	              0
  retrieve shop                       	          2	          2	              0

# api.benchmark product
  test name                           	left count 	right count	duplicate count
  ------------------------------------	-----------	-----------	---------------
  product details                     	         13	         13	              3

# api.benchmark variant
  test name                           	left count 	right count	duplicate count
  ------------------------------------	-----------	-----------	---------------
  retrieve variant list               	         15	         15	              8

[codecov]

Codecov Report

Merging #4709 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4709      +/-   ##
==========================================
+ Coverage   91.59%   91.62%   +0.03%     
==========================================
  Files         308      308              
  Lines       18438    18438              
  Branches     1827     1827              
==========================================
+ Hits        16888    16894       +6     
+ Misses       1041     1038       -3     
+ Partials      509      506       -3

Impacted Files	Coverage Δ
saleor/order/models.py	90.94% <0%> (+0.78%)	⬆️
saleor/payment/utils.py	92.03% <0%> (+1.59%)	⬆️

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