[HTTPS-Everywhere] Replacing the rulesets database file

[Jacob S Hoffman-Andrews]

I think we should not try to ship a different format for updates 
than we ship with releases. I think we're likely to wind up with 
people whose DBs are in strange intermediate states that are broken 
and hard to debug. The rulesets.sqlite file is small enough that 
it's reasonable to always ship the whole thing.

It looks like arraybuffers are what you want to retrieve binary data 
using XHR: 
https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/Sending_and_Receiving_Binary_Data.

I'll take a look at your changes in terms of loading the rulesets DB 
from the user's profile directory. It sounds like you're probably on 
the right track. I was expecting that we'd need to make a change 
somewhat like that.