Subversion 1.12 release preparation

It's March now; Subversion 1.12 is due out in April. To give us the standard 4 weeks to stabilize, it's now time to make a branch and a release candidate and do all the usual updates associated with release management.

I'll get started.

If there's anything I ought to take into account, do speak up.

Julian Foad wrote on 2019-02-07:
> [...] I'm going to be unavailable around the end of March to end of April. [...]
>
> Could someone else do the final stages of the release management in April?

It would be nice to have a back-up for that. I might end up having some availability then, but I just don't know in advance.

-- 
- Julian