Wagtail 4.0 support

[katdom13]

• Updated to support wagtail 4
• Dropped support for versions of Wagtail less than 2.15
• Updated documentation
• Added custom template for checking wagtail routes (e.g. wagtailsnippets)
• Fixed imports
• Fixed formatting
• Added tox.ini
• Updated ci testing with tox to properly manage versions

Found 56 test(s).
Creating test database for alias 'default'...
System check identified some issues:

WARNINGS:
tests.Advert.publications: (fields.W340) null has no effect on ManyToManyField.

System check identified 1 issue (0 silenced).
.......................................................4 object created. 0 object updated. 0 object skipped.
4 items created. 0 items updated. 0 items skipped.
.
----------------------------------------------------------------------
Ran 56 tests in 0.334s

OK

[jacobtoppm]

Hey, apologies this was mixed @katdom13! Would you be able to fix conflicts and then we can get this merged?

[katdom13]

Hi @jacobtoppm , I have fixed the conflicts. Can you please approve the workflow approval for this? Thank you!