Fixed the internal error filtering pages by URL in the dashboard 1.0

[NyanKiyoshi]

Closes #4585.

Screenshots

Pull Request Checklist

1. Privileged views and APIs are guarded by proper permission checks.
2. All visible strings are translated with proper context.
3. All data-formatting is locale-aware (dates, numbers, and so on).
4. Database queries are optimized and the number of queries is constant.
5. Database migration files are up to date.
6. The changes are tested.
7. GraphQL schema and type definitions are up to date.
8. Changes are mentioned in the changelog.

[django-queries]

Here is the report for 4e48522 (NyanKiyoshi/saleor @ fix/dashboard-1.0/pages-filters)
Base comparison is e44fcd5.

No differences were found. (click me)

# api.benchmark checkout
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  add billing address to checkout            	         38	         38	             20
  add shipping to checkout                   	          7	          7	              0
  checkout payment charge                    	         14	         14	              0
  complete checkout                          	          6	          6	              0
  create checkout                            	         52	         52	             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                            	         15	         15	              3
  retrieve product attributes                	         13	         13	              2

# api.benchmark variant
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  retrieve variant list                      	         18	         18	              8

# api product sorting attributes
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  sort product not having attribute data     	         21	         21	              0

[codecov]

Codecov Report

Merging #4776 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4776      +/-   ##
==========================================
+ Coverage    91.2%   91.21%   +0.01%     
==========================================
  Files         343      343              
  Lines       20417    20415       -2     
  Branches     1937     1936       -1     
==========================================
+ Hits        18621    18622       +1     
+ Misses       1259     1256       -3     
  Partials      537      537

Impacted Files	Coverage Δ
saleor/dashboard/page/filters.py	100% <100%> (+13.33%)	⬆️
saleor/dashboard/discount/forms.py	97.64% <0%> (-0.06%)	⬇️
saleor/page/models.py	87.75% <0%> (+2.04%)	⬆️

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