Provide fake username on login form

[yubiuser]

What does this PR aim to accomplish?:

When a password is set for the web interface, we use autocomplete="current-password" on the password login form to allow browsers and password managers to auto-fill the input field. This works well with e.g. Firefox, but fails on Chrome (see #2418).
To fix this, we add a hidden input field providing a fake username (pi.hole).

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[yubiuser]

Note to self: get rid of Chrome's autofill CSS modifications of the input box

[rdwebdesign]

This PR will solve the issue, but I think this is a browser bug introduced recently.

To be sure, I tested using the current browser version and 2 different Pi-hole versions (docker image latest and 2022.02) and the password is not saved in both cases.

Without your PR, I was able to save the password on both browsers using the normal procedure, but I needed to type an username:

• access the page, type the password and click on "Log in" button;
• the browser will show a pop up to save the password;
• type any username and click "save";
• the password will be saved.

[yubiuser]

Do you think I should not proceed with this PR as this is an external "bug" (and we could simply wait until they fix it)? Maybe they don't see this as a bug, but as a feature and won't fix it ever?

[yubiuser]

Ready for review.

I adjusted the CSS to overwrite the auto-fill style on Chrome browsers.

[pralor-bot]

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

https://discourse.pi-hole.net/t/pi-hole-ftl-v5-20-1-web-v5-18-1-and-core-v5-15-released/60495/1