Backport source links from 3.x.

Existing links have been updated to use the new reST role.  In some
files, I have also made cosmetic changes to the header.
diff --git a/Doc/library/imaplib.rst b/Doc/library/imaplib.rst
index 95bdc70..9fcbaaa 100644
--- a/Doc/library/imaplib.rst
+++ b/Doc/library/imaplib.rst
@@ -16,6 +16,10 @@
    pair: IMAP4_SSL; protocol
    pair: IMAP4_stream; protocol
 
+**Source code:** :source:`Lib/imaplib.py`
+
+--------------
+
 This module defines three classes, :class:`IMAP4`, :class:`IMAP4_SSL` and
 :class:`IMAP4_stream`, which encapsulate a connection to an IMAP4 server and
 implement a large subset of the IMAP4rev1 client protocol as defined in