Allow links to be created/updated/deleted from UI

[fxnn]

A user of the UI wants to create a link from one path to another path inside the Gone content root.
A user of the UI wants to edit a link's target, if he has the permission to do so.
A user of the UI wants to delete a link, if he has the permission to do so.

Background

Currently, Gone supports symlinks that are handled transparently, i.e. a user can view/edit the symlinked file reps. access the symlinked directory without seeing its real name -- or even knowing that there's a symlink. It is not possible to CRUD symlinks from the UI.

Now, Gone also needs a link mechanism that one can handle from the UI. I don't think that we should use symlinks here, as they have one problem: they don't have their own set of permissions; instead, the operating system always displays them as rwxrwxrwx. This is not practicable for Gone, because we need to know: who has the privileges to edit the link?

In the end, we need a link mechanism implemented as regular files, identified by a special suffix (e.g. .link or .redirect). I wonder whether we should use transparent links or intransparent redirects, i.e. whether we should just deliver the linked content und the link's URL, or whether we should redirect (307 Temporary Redirect) and therefore display the new URL to the user.

[fxnn]

Deleting files and directories in general is handled in #6 (Allow for deleting files an directories).