Attachments: Allow user-chosen directory and inheritance

Attachments: Allow user-chosen directory and inheritance

The directory used for attachments to an entry is by default chosen
automatically, with a name derived from the entry ID to make sure it
is unique.

However, in some cases it may be better to assign a user-chosen
directory to and entry, to be used for attachments.  This commits
implements this change.  See the documentation changes for information
on how things work.

Thanks to Jason Jackson for pushing this part.

The second change made bu this commit is that it is now possible to
inherit the attachment directory from a parent, so that an entire
project subtree can use a single directory.