Create PR template for news items and blogs

[sdruskat]

To make sure they follow the correct categories, etc.

[nuest]

Depends on #170 - i.e. we need a working categorisation first to include it in the PR template.

Agreed @sdruskat ?

[sdruskat]

Yes. I'm not sure if we can split news and blog posts as nicely as we thought, may need some Liquid logic and hard separation.