Don't open webtorrent in tor windows

[tildelowengrimm]

Using the BitTorrent protocol over Tor results in poor performance and poor privacy. When using Tor, .torrent files should not be opened with webtorrent — instead, they should just be downloaded like any other file. Magnet links should result in a privacy warning. If the user choses to proceed, they could either be opened in-browser or in a desktop client. To reduce linkability, perhaps, Magnet links should be added to webtorrent but not opened until a short while after the Tor session ends

[srirambv]

.torrent files open webtorrent on Tor windows.

[diracdeltas]

Changing this issue to reflect that webtorrent should not be enabled for Tor at all

[riastradh-brave]

Marking this as a release blocker since torrents are a leak and this would be a regression from browser-laptop. Looks like @yrliou is on top of it, will take a look when the PR is ready.

[yrliou]

QA note: See brave/brave-core#554 for the test plan.

[btlechowski]

Verification passed on

Brave	0.55.12 Chromium: 70.0.3538.45 (Official Build) (64-bit)
Revision	cbdc32e4334458954e9def214d7e5fa1ca1960eb-refs/branch-heads/3538@{#830}
OS	Windows 7

Used test plan from brave/brave-core#554

Verification Passed on

Brave	0.55.12 Chromium: 70.0.3538.45 (Official Build) beta (64-bit)
Revision	cbdc32e4334458954e9def214d7e5fa1ca1960eb-refs/branch-heads/3538@{#830}
OS	Linux

Went through verification using the following build under macOS 10.13.6 x64 - PASSED

Brave	0.55.12 Chromium: 70.0.3538.45 (Official Build) beta(64-bit)
Revision	cbdc32e4334458954e9def214d7e5fa1ca1960eb-refs/branch-heads/3538@{#830}
OS	Mac OS X

[kjozwiak]

@yrliou when there's no applications associated with magnet links, clicking on a magnet link won't do anything under Tor, meaning there's no feedback. I think we should add some type of warning/modal window that tells the user that webtorrent isn't supported under Tor if the user doesn't have a third party application installed.

Here's an example of what we did with browser-laptop (muon):

Thoughts?

[tildelowengrimm]

What happens when you click a link which has no handler installed in general?