Refactored `elwiki/commit-log'

Separated the null-separated record to alist mapping code into a
separate function, `elwiki/log->alist'.
Refactored header/footer sending code

Refactored the mostly duplicate code in `elwiki/render-page' for
sending the site-wide header and footer into a separate function,
`elwiki/send-site-header-or-footer'.