Big board CSS generator should skip liveblog

[alykat]

Right now, the script (fab utils:create_board_overridecss) loops over everything in the nav sheet and generates a big board-style override for every Seamus ID in the list. That includes the liveblog. However, the liveblog uses a different template (the standard story template rather than the wide multimedia template), and its overridecss is backed up in the liveblog repo. Having the version in the election18-graphics repo means that it's easy to accidentally overwrite existing liveblog styles.

Ideally, the process that generates the board css files would be able to check that the slug isn't "liveblog" before generating the css. (It appears to only look at Seamus IDs, not any of the other columns.)

Not a big deal, but something to be aware of / look into for the next time this feature is used.