Remove dedicated black/whitelist pages + redesign menu

[yubiuser]

• What does this PR aim to accomplish?:

This PR removes the separate Black/Whitelist pages from the nav menu. They have been there to aid new users and to make the transition pre-v5 -> v5 more easy. It was always planed to remove them at some point.
They have been only a font-end for groups-domais.php where the group column was hidden.

The whole functionality can still be accessed via Group Management -> Domains.

After removing the two entries the menu looked quite empty so I re-structured it a bit. Additional, added a link to our Discourse forum.

• How does this PR accomplish the above?:

Remove now unused code.

---

By submitting this pull request, I confirm the following:

1. I have read and understood the contributors guide, as well as this entire template. I understand which branch to base my commits and Pull Requests against.
2. I have commented my proposed changes within the code and I have tested my changes.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)

---

• I have read the above and my PR is ready for review. Check this box to confirm

[yubiuser]

Proposed new menu structure

Logged out:

Logged in:

Without password set:

[DL6ER]

I propose removing "GROUP" from "GROUP MANAGEMENT", see what I added for branch new/FTL_is_my_new_home:

[yubiuser]

That's where I took it from but deliberately added the Group because all the sub menu items deal with the group management. Also management alone is very close to System or Settings

[yubiuser]

Ready for review. I guess there might be more code that could go...

[rdwebdesign]

There is something wrong with the "Collapsed" menu.

I can look into it later (or tomorrow).