Absolute urls in local editor links

[glanzel]

This commit solves the Issue #8
It changes only the urls of links created in the birdsong editor
all other Richt Text Field Links are left untouched.

I hope that could be a useful approach for the Issue.

[seb-b]

Sorry missed this when it was first opened.

This looks good, I hadn't seen register_temporarily before. Looks like it was introduced in wt 2.10 so we'll have to up the minimum version.
I cloned this branch and gave it a go, but my link added was still relative, was this working for you?

[glanzel]

Hi I just tested it again. Still works for me.
I use it like that:
`class ACampaign(Campaign):
body = StreamField(DefaultBlocks())

panels = Campaign.panels + [
    StreamFieldPanel('body'),
]`

When i use PageLinks or DocumentsLinks in the DefaultBlocks Editor the Preview already shows an absolute path for me.