Everywhere that REMOTE_ADDR was used, a session object is available, so
instead use its remote_addr method.
In IkiWiki::Receive, stop setting a dummy REMOTE_ADDR.
Note that it's possible for a session cookie to be obtained using one IP
address, and then used from another IP. In this case, the first IP will now
be used. I think that should be ok.
Always pass the full (modified) content in `content` named parameter. When the
user edits an existing wiki page, also pass a `diff` named parameter, which
includes only the lines that they added to the page, or modified.
Signed-off-by: intrigeri <intrigeri@boum.org>