enable auto-correction

[andreasgal]

Don't merge yet. Incomplete.

Issues:

• disabling auto correct with a single backspace seems suboptimal (disables too easily)
• prediction engine doesn't predict words where the first character is a near-hit (s -> a)

[andreasgal]

@ckerschb @gregorwagner

[andreasgal]

(feel free to steal, by the way)

[andreasgal]

Before I forget, the other .dict files need to be re-created, too. I only updated en-US.

[andreasgal]

I tried to investigate the issue why "s" is not predicted as "a", even though its a nearby key. I think this is some algorithmic problem in prediction.js. The new DAG code is sufficiently different from my bloom filter approach that I couldn't find a way to fix this. @ckerschb please help.

[davidflanagan]

As suggested above, I'm going to go ahead and steal this, and work on it under https://bugzilla.mozilla.org/show_bug.cgi?id=797170

I'll probably start a new pull request that borrows liberally from this one.

[mozilla-autolander-deprecated]

This pull request has been closed due to tree stability issues. Please rebase and re-open the pull request if you still need to land this. Ensure the gaia-try run is green before landing. Sorry for any inconvenience.