Support our own hotfix mechanism

We disabled the Firefox Hotfix system in legacy/trac#16837 (moved) over concerns about compatibility/conflicts.

We can enable this again by setting extensions.hotfix.url to our own servers, and only pushing out a specific Mozilla hotfix after we've tested it on TBB ourselves, but then we need to deal with pinning and other issues. The pinning for AMO is specified here: https://mxr.mozilla.org/mozilla-central/source/security/manager/tools/PreloadedHPKPins.json#201

Mozilla is also planning on replacing the Hotfix mechanism at some point, so we'll need to watch for falling bits.