Figure out "series realms" (pages that are derived from a series)

[LukasKalbertodt]

In most use cases, most realms are likely just there to show a single series. We can of course achieve this by having a series block, but this is not ideal, as the realm title does not update when the series is updated, for example. And also, it means we have explicit series blocks in the DB.

An alternative would be to have "series realms" that do not explicitly store these content blocks, but "generate" them on the fly/implicitly. That would mean that description and title are always automatically updated when the series changed.

Open questions:

• How to exactly store this in the DB? We likely still want a row in realms to be able to use columns like index.
• What about path segments? Derived from series title? But that might not be optimal and result in ugly long paths. Also, it is not supposed to change easily as it invalidates links. So do we not update it on title changes?
• Can these realms have children?
• Are these realms converted to normal realms when users try to edit their content? But then we need to warn and explain to the user that this means losing these auto-update features.
• OR maybe we want to do it on block-level or title-level instead of realm level? So that there is a "series description" block always getting up to date information? And for realms, there is just the option to have the title always be equal to the title of a series?

[LukasKalbertodt]

We decided to just go with the ability to derive the realm name from a block in that realm. Having "series realms" be something completely different is bad in a few ways, mostly confusing users. And it would mean that adding a single text block breaks the "automatic name updates". So the idea is: have a normal realm, with a series block and the name derived from that block.

This was implemented in #469