Previously with the default template settings (`indent` variable not set), we would get interparagraph spaces separating bib entries even with `entry-spacing="0"`. On the other hand, setting `entry-spacing="2"` gave ridiculously large spacing. This change makes the spacing caused by `entry-spacing` a multiple of `\parskip` by default, which gives aesthetically reasonable output. Those who want a larger or smaller unit (e.g. because they use `indent` which sets `\parskip` to 0) may `\setlength{\cslentryspacingunit}{10pt}` in header-includes to override the defaults. Closes #7296.