Improve custom DNS hosts file parsing

[NickRF]

What does this PR aim to accomplish?:

Improve parsing of custom DNS hosts file. This is useful when directly importing an existing hosts file in to pihole, which may contain comments or variable whitespace.

This allows entries with variable whitespace to appear in the web interface.

How does this PR accomplish the above?:

Modify parsing code to allow variable whitespace, and exclude comments

Link documentation PRs if any are needed to support this PR:

n/a

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[yubiuser]

Thanks for your PR. We are in the middle of the development-v6 > development merge for the upcoming Pi-hole v6. Please base and target your PR on the v6 branch.

[NickRF]

This change isn't relevant to the development-v6 branch.