feat(district): rework election page tree from scratch, update to latest styleguide 2.10.0, load styleguide from CDN and more

This makes election page tree structure more flexible, as it allows for multiple candidacies within one root election campaign. E.g. each city part having it's own election process. Especially useful for big cities.

Note: I had to do some migration tweaks, Wagtail seems to be a little fragile. These are following:

• Manual edit of district/migrations/0053_alter_districtcrossroadpage_cards_content.py to remove references to now not-existing page types.
• Data migration to delete all the superseded election page types. Be careful when migrating. Make sure you understand that this will delete some data!

Besides election pages, I also:

• Upgraded to latest styleguide 2.10.0
• Linked styleguide from CDN and modified references to --image-url to use full path (otherwise, it would be sourced from styleguide domain)
• Added support for candidates from other parties
• Fixed broken person page when person had no social media stuff (#108 (closed))
• Made person email not mandatory and consider show_email toggle wherever applicable (#107 (closed))