Support Wagtail 4.2 (Current deprecation warnings)

[Timusan]

The current version of Wagtail Localize uses imports deprecated since Wagtail 4.2:

• importing from wagtail.core.blocks has been deprecated in favor of wagtail.blocks
• importing from wagtail.core.models has been deprecated in favor of wagtail.models
• importing from wagtail.core.rich_text has been deprecated in favor of wagtail.rich_text
• importing from wagtail.admin.edit_handlers has been deprecated in favor of wagtail.admin.panels
• importing from wagtail.core.signals has been deprecated in favor of `wagtail.signals``

This gives a large amount of warnings on startup. I see a PR is underway, did it take this into account as well?

[zerolab]

@Timusan #673 will take care of that. e.g. https://github.com/wagtail/wagtail-localize/pull/673/files#diff-169de4ea946b4fae4e8f1f45e566d0b96886b2cf502557b21ae36228db1bf3ceL14

On my list to re-review and get in as soon as possible

[zerolab]

Fixed by #673. Release imminent