Create correct links to older version of attachments when viewing a revision

When viewing an old revision of a document we need to link to attachment from this time. If the attachment has been deleted a nice error message needs to be shown.

Steps to reproduce:

1/ Create a document in v1.2
2/ Attach a file v1.1 (document becomes 1.3)
3/ Modify document v1.4
4/ Update the file v1.2 (document becomes v1.5)
5/ Modify document v1.5
6/ View revision 1.2 (no attachment list should be shown)
7/ View revision 1.3 (attachment should link to file 1.1)
8/ View revision 1.5 (attachment should link to file 1.2)
9/ Delete attachment
10/ View revision 1.2 (attachment link should show an error stating attachment has been delete since this version)