Simplify domain lists, remove a lot of duplicated code

[DL6ER]

By submitting this pull request, I confirm the following:

• I have read and understood the contributors guide, as well as this entire template.
• I have made only one major change in my proposed changes.
• I have commented my proposed changes within the code.
• I have tested my proposed changes, and have included unit tests where possible.
• I am willing to help maintain this change if there are issues with it later.
• I give this submission freely and claim no ownership.
• It is compatible with the EUPL 1.2 license
• I have squashed any insignificant commits. (git rebase)

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What does this PR aim to accomplish?:

Simplify domain management by using the same page for both locations where domains can be edited. This removes a large amount of duplicated code, each prone to its own bugs.

Some modifications ensure that features like automatic conversion to the legacy "wildcard" style. This is only available on the "simpler" pages to not confuse users (this happened a few times in the past).

How does this PR accomplish the above?:

See above.

What documentation changes (if any) are needed to support this PR?:

None

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/restore-ability-to-add-blacklists-in-bulk/28091/22