make blocklist URL configurable

[charles]

Hosting the blocklist on transmissionbt.com has several problems:

• We need to reduce the bandwidth used on transmissionbt.com by the blocklist.

• Lack of configurability -- several users have wanted to use their own blocklist.

• Having a single, non-configurable blocklist might wrongly give users the impression that we've reviewed and/or endorse the contents of that blocklist.

• Having a single, non-configurable blocklist might wrongly give users the impression that the blocklist they're getting is a "silver bullet" for blocking out peers from spyware/malware addresses.

We can address all of these issues by making the blocklist configurable and not recommending any default blocklist.

[charles]

implemented for GTK+ and Qt client

[charles]

implemented for GTK+, Qt, and RPC in r11368

[livings124]

r11369 for Mac

[Rolcol]

Are you guys open to the idea of having the update interval be configured by the user?

[livings124]

that seems one too-many options