Admin changes for #5046

- Removed post type classes. Since they were always loaded, there was
no benefit having them split up. Combined meaning less hooked in
methods.
- Stopped loading some function files and sessions in the admin area.
They are not needed there.
- Made a few admin page classes static as they are stateless
Merge pull request #5614 from mkayan-wakefly/master

RE issue: #5613 - Typo in error message
unslash order item meta key before updating in DB

To prevent nested slashes in DB when item meta key contains quotes.
This is in line with WordPress' behaviour - see update_meta() in wp-includes/meta.php
added a notice to display messages about available translation upgrades
added a new option in system status to force the translations upgrade
Update readme.txt

Spelling
Allow downloadable products URL to be relative
Merge pull request #5621 from tivnet/patch-1

Update readme.txt
Merge pull request #5619 from claudiosmweb/feature-system-status-languages

Add a new option in system status to force the translations upgrade
Merge pull request #5618 from claudiosmweb/feature-translation-update-notice

Add a notice about available translation upgrades
Placeholder functions
Merge pull request #5598 from splashingpixels/add-filter

add filter for admin stock report products per page
Merge pull request #5602 from getdave/master

Ensure script is safe when global var not available/enqueued
Merge pull request #5605 from ragulka/radio-field

Add radio type support to woocommerce_form_field
Merge pull request #5610 from ragulka/checkout-wc-clean-fix

Check if posted data is_array before doing wc_clean in checkout
Merge pull request #5617 from enru/master

unslash order item meta key before updating in DB
Add filter to allow override the cache widget id
Removed ununsed global variables in local scopes
Remove ordering query args

If these are not removed right after use they will affect the remaining
queries on the page.
Merge pull request #5638 from coenjacobs/code-cleanup

Removed unused global variables in local scopes