Enable/disable horizontal comments via JS not server side

Revert most of "Add URL param `2d=true/false` that enables/disables
horizontal comments." 22ab81275cc89e19206a83f892f1007ed2875a9c. It
doesn't work because the rendered page is cached. Implement the same
functionality in Javascript instead.
Change body and comment ids from 65502, 1, 2, 3, ... to 1, 2, 3, 4, ...

And change title id from 65501 to 0.
Generate numeric sequential page ids

And add generate-next-page-id database function.
Adjust QuotaChargerSpec limits to make the tests work

And use nextPageId().
Fix broken test, convert another from Specs to ScalaTest.
Add test cases for PagePath and "/-pageid-slug" page names.
Use '/-pageid/slug' instead of '-pageid-slug', add tests.
Merge branch 'topic-discourse-compatibility'.
Add /-/create-forum endpoint.
Don't require any /_site.conf.
Use '?siteType=...' URL param instead of cookie

So it's possible to remember site type across browser restart.
Also remove site name cookie and pointless create-site GET request.
Fix site logo HTML bug, and set default value to 'Home'.
Rename PagePath.path to .value.