Fix/settings panel append

[benfroelich]

see issue 440

Changes to docs/source/contributing/index.rst are cosmetic and not related; I can revert if desired. I thought it was easier to read this way using shell expansion.

[benfroelich]

when running tests there are several build failures related to incompatible django/wagtail versions, e.g. wagtail 2.15 depends on Django<3.3 and >=3.0 when I run tox -e py310-dj41-wt215 these show up in travis as coverage not allowed unfortunately.

[MrCordeiro]

Hey, Ben sorry for the delay ＞︿＜

I find your PR pretty complete. It's great, thanks!

The only thing I ask is for you to rebase your fork so that we get rid of merge conflicts.

[djch]

If this fixes the remaining WARNINGs, I'd be glad to see this merged!

wagtailmenus.FlatMenu: (wagtailadmin.W002) FlatMenu.content_panels will have no effect on modeladmin editing
        HINT: Ensure that FlatMenu uses `panels` instead of `content_panels`or set up an `edit_handler` if you want a tabbed editing interface.
There are no default tabs on non-Page models so there will be no Content tab for the content_panels to render in.
wagtailmenus.FlatMenu: (wagtailadmin.W002) FlatMenu.settings_panels will have no effect on modeladmin editing
        HINT: Ensure that FlatMenu uses `panels` instead of `settings_panels`or set up an `edit_handler` if you want a tabbed editing interface.
There are no default tabs on non-Page models so there will be no Settings tab for the settings_panels to render in.
wagtailmenus.MainMenu: (wagtailadmin.W002) MainMenu.content_panels will have no effect on modeladmin editing
        HINT: Ensure that MainMenu uses `panels` instead of `content_panels`or set up an `edit_handler` if you want a tabbed editing interface.
There are no default tabs on non-Page models so there will be no Content tab for the content_panels to render in.
wagtailmenus.MainMenu: (wagtailadmin.W002) MainMenu.settings_panels will have no effect on modeladmin editing
        HINT: Ensure that MainMenu uses `panels` instead of `settings_panels`or set up an `edit_handler` if you want a tabbed editing interface.
There are no default tabs on non-Page models so there will be no Settings tab for the settings_panels to render in.

[MrCordeiro]

LGTM!
Nice work, @benfroelich!