Remove '_site.conf' and theme config page

This might make it impossible to install new Debiki servers, fix later.
Hide dangerous 'View page settings' link, except for my own sites.
Add logoUrlOrHtml site setting

Use it instead of `tpi.websiteConfigValue("html-logo-slogan"))`.
Show the logUrlOrHtml setting in a <textarea>.
Security bug fix: Add access control to SettingsController.
Redirect to admin login page if not logged in.
Tweak title editor margins and height

Don't show more than one line: don't set height.
Add URL param `2d=true/false` that enables/disables horizontal comments.
Let the default theme use font 'Bree serif', tweak top bar CSS.
Add link from admin-dart to site homepage, fix link to admin-dart.
Always show forum body, by default.