Change “page” templates to “xhtml”

Merge template types “page” with “xhtml”; there’s no distinction between
the two types anymore.
Add database_unavailable.xhtml template

Used by QlDb, updated to use this new template name.
Migrate QlSession::require_priv_tokens() to use QlErrorResponse

Drop old ad-hoc error output code for the new generic QlErrorResponse,
which also allows to use a template (new access_denied.xhtml).