feat(boardsv2): hide flagged comments

[x1unix]

Description

This PR adjusts display of flagged comments with flow defined in #3480.

Now, flagged comments will be hidden but children replies still be visible.

Demo

Important

The "comment is hidden" message is actually italic, but italic font styles are broken in Gnoweb

Other Changes

In addition to that:

• Fixed Reply URL arguments.
• Added Flag thread/comment URL.
• Started adopting strings.Builder instead of string concatenation.

CC @jeronimoalbi

Closes: #3480

[Gno2D2]

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