Add configurable htpasswd location

[dwmunster]

What is the purpose of this change? What does it change?

Allows users to configure the location of the .htpasswd file.

Was the change discussed in an issue or in the forum before?

Closes #187

Checklist

• [x] I have enabled maintainer edits for this PR
• [x] I have added tests for all changes in this PR
• [x] I have added documentation for the changes (in the manual)
• [x] There's a new file in changelog/unreleased/ that describes the changes for our users (template here)
• [x] I have run gofmt on the code in all commits
• [x] All commit messages are formatted in the same style as the other commits in the repo
• [x] I'm done, this Pull Request is ready for review

[dwmunster]

Assuming that updating the readme suffices for updating documentation, I have marked that complete as well. If there is additional documentation needed, I would be glad to add that.

[remram44]

Perhaps a new image should be published with that feature?