Format index.html source

[dr-shorthair]

The indentation in ssn/index.html is currently inconsistent, which makes editing more difficult. Various tools can fix this automatically (e.g. VSCode plugins). However, this trivial (non-normative) action will change a large number of lines in the file so will look like a big diff.

Propose to do it once quickly when we have few branches.

[dr-shorthair]

Also fix

• buttonpanels throughout
• some section and div misbalances
• some header levels