(bug 18174) Added Older and Newer change link to Abusefilter diff

On top of that I changed /history to no longer show the
changes link if it's the first revision of the filter.

As I was on it I fixed the diff selection for diffs like
history/1/diff/2/next where change number 2 was compared
to the last change of the filter instead of the next one.
This probably went unnoticed because the /next action isn't
linked anywhere within the software itself.

Change-Id: Ic28bb696a4dbbb75e0bfcae1843ccefab1a9508a
Merge "(bug 18174) Added Older and Newer change link to Abusefilter diff"