In Rails 5 the `per_page` argument came from `params` is String but an
Integer is expected.

This commit explicitly converts `per_page` to an Integer.
It fixes the errors like:

```
  1) Groups::ChildrenController GET #index for subgroups filtering children succeeds if multiple pages contain matching subgroups
     Failure/Error:
       hash[page] = second_collection.page(second_collection_page)
                      .per(per_page - paginated_first_collection(page).size)
                      .padding(offset)

     NoMethodError:
       undefined method `-' for "3":String
       Did you mean?  -@
     # ./lib/gitlab/multi_collection_paginator.rb:42:in `block in paginated_second_collection'
     # ./lib/gitlab/multi_collection_paginator.rb:46:in `paginated_second_collection'
     # ./lib/gitlab/multi_collection_paginator.rb:14:in `paginate'
     # ./app/finders/group_descendants_finder.rb:52:in `children'
     # ./app/finders/group_descendants_finder.rb:34:in `execute'
     # ./app/controllers/groups/children_controller.rb:36:in `setup_children'
     # ./app/controllers/groups/children_controller.rb:18:in `index'
     # ./lib/gitlab/i18n.rb:50:in `with_locale'
     # ./lib/gitlab/i18n.rb:56:in `with_user_locale'
     # ./app/controllers/application_controller.rb:351:in `set_locale'
     # ./spec/controllers/groups/children_controller_spec.rb:182:in `block (5 levels) in <top (required)>'
     # ./spec/spec_helper.rb:198:in `block (2 levels) in <top (required)>'
```

Resolve "Wiki headings no longer show anchor link"

Closes #44956

See merge request gitlab-org/gitlab-ce!18591

ShaAttribute crashes with ArgumentError if column doesn't exist

See merge request gitlab-org/gitlab-ce!18880

Update gon ~> 6.1.0 to ~> 6.2

See merge request gitlab-org/gitlab-ce!18764

Fix finding wiki pages when they have invalidly-encoded content

Closes #43715

See merge request gitlab-org/gitlab-ce!18856

Hide merge request option in IDE when merge requests are disabled

Closes #45698

See merge request gitlab-org/gitlab-ce!18857

Fix outdated web ide copy

See merge request gitlab-org/gitlab-ce!18861

Merge branch '41229-include-the-human-repo-name-in-gitlab-shell-log-when-hashed-storage-is-in-use' into 'master'

Resolve "Include the human repo name in gitlab-shell.log when hashed storage is in use"

Closes #41229

See merge request gitlab-org/gitlab-ce!18870

Support resetting of Prometheus metrics between test runs

Closes #39968

See merge request gitlab-org/gitlab-ce!18836

Remove accidental EE additions in CE

See merge request gitlab-org/gitlab-ce!18877

Improve fix for intermittent QA failure in "cloning code using a deploy key"

Closes charts/gitlab#396

See merge request gitlab-org/gitlab-ce!18608

Resolve "Update `updated_at` on an issue when label or time spent was added"

Closes #45442

See merge request gitlab-org/gitlab-ce!18757

[CE] Backport with_forgery_protection spec helper

See merge request gitlab-org/gitlab-ce!18833

Backport some changes from gitlab-ee!5476

See merge request gitlab-org/gitlab-ce!18760

Resolve "NoMethodError: undefined method `uploader_context' for #<ObjectStorage::MigrateUploadsWorker::Upload:0x00007f59e..."

Closes #46147

See merge request gitlab-org/gitlab-ce!18820

This is especially helpful when hashed storage is enabled

Replace the `project/issues/references.feature` spinach test with an rspec analog

See merge request gitlab-org/gitlab-ce!18769

Replace the `project/deploy_keys.feature` spinach test with an rspec analog

See merge request gitlab-org/gitlab-ce!18796

Adding the :prometheus tag to an rspec test will clear out
memory-mapped files and reset the registry.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/39968

refactor 'cycle_analytics' to use axios

See merge request gitlab-org/gitlab-ce!18782

Add Repository#xcode_project? method

See merge request gitlab-org/gitlab-ce!18854

The lib/gitlab/git/repository.rb needs to have the same content between
gitlab-ce and gitlab-ee in order to have Gitaly working fine.

refactor "deploy_keys" to use axios

See merge request gitlab-org/gitlab-ce!18792

Fix project mirror data schema

See merge request gitlab-org/gitlab-ce!18811

Removed the d3_rails gem

See merge request gitlab-org/gitlab-ce!18860

# Conflicts:
#   db/schema.rb

Update documentation to use inclusive language, gender neutral name and pronoun.

See merge request gitlab-org/gitlab-ce!18858

Resolve "Incorrect documentation for .gitlab-ci.yml cache:key default value"

Closes #45840

See merge request gitlab-org/gitlab-ce!18841