document manual server-on-.onion Tor setup process

[warner]

Now that #2788 has landed, one remaining task for #517 (Tor support) is to explain how to setup Tor-only servers. The process is:

• choose LOCAL port, choose TORPORT numbers
• configure Tor to point an onion address (+TORPORT) at localhost:LOCAL
• restart Tor, learn the ONION address
• edit tahoe.cfg to set tub.port to tcp:LOCAL:interface=127.0.0.1
• set tub.location to tor:ONION.onion:TORPORT
• maybe set anonymous=true (#1010)

We should also include some docs on how to test that it's working. The welcome page shows what IP address is used for current connections: it should be localhost (for outbound SOCKS connections, or inbound connections from the Tor daemon that originated as .onion connections).

It should also have some pointers on how to install the necessary libraries: pip install tahoe-lafs[tor], and apt-get install tor, or running the Tor Browser Bundle.

[Brian Warner <warner@…>]

In 327bef3/trunk:

update tor/i2p docs to match current config syntax

This removes the section that describes automatic configuration using
transport-agnostic endpoint-centric tub.port strings. That was the
approach where tub.port used "onion:80:hiddenServiceDir=PATH", and
Foolscap was able to query the generated Listener to find out what
address it was supposed to advertise. We considered this for a long
time, but in the end decided to use a more static approach, where
foolscap/tahoe never try to guess it's location: Tahoe always requires
tub.location= to be set.

When we get automatic configuration implemented, it'll be a simple CLI
argument, something like "tahoe create-server --listen=tor".

Instead, this document now explains how to configure Tor to create the
hidden service, then how to copy the generated .onion address into the
tahoe config.

This also removes a lot of other text that seems irrelevant now, and
refers the user to the tahoe.cfg docs (configuration.rst) instead of
including all the [tor]/[i2p] docs inline.

Closes ticket:2815