Document "forget" security considerations and add references

Removing data based on a policy when the attacker had the opportunity to
add data to your repository comes with some considerations. This is
added to the 060_forget.rst documentation.

That document is also updated to reflect that restic now considers
the current system time while running "forget".

References to the security considerations section are added:
- In `restic forget --help`
- In the threat model (design.rst)
- In the (030) setup section where an append-only setup is referenced

A reference is also to be added to the `rest-server` readme's
append-only paragraph (see my fork).

This commit also resolves a typo (amount->number for countable noun),
changes a password length recommendation into the metric that
actually matters when creating passwords (entropy) since I was editing
these doc files anyway, and updates the outdated copyright year in
`conf.py`.

Some wording in 060_forget (line 21..22) was changed to clarify what
"forget" and "prune" do, to try and avoid the apparent misconception
that "forget" does not remove any data.

cmd/restic/cmd_forget.go

@@ -18,6 +18,8 @@ The "forget" command removes snapshots according to a policy. Please note that
 this command really only deletes the snapshot object in the repository, which
 is a reference to data stored there. In order to remove this (now unreferenced)
 data after 'forget' was run successfully, see the 'prune' command.
+When using this command on special append-only repositories, please see the
+documentation for security considerations.
 
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