Add advanced setting to enable editing nonce on confirmation screens

[danfinlay]

This is a specific solution proposal that fixes #1410, for the sake of possibly adding a clean and simple bounty to.

Scope:

1. A new setting should be added to app/scripts/controllers/preferences.js for whether a user wants a nonce field.
2. This setting should be exposed as a toggle switch in the settings screen as "Show nonce field on confirmation screens. Only use if you understand the implications." (This message should be written in the i18n file).
3. If this boolean is true, render a field on the tx confirmation screen that allows hard-coding this otherwise dynamically computed value. The placeholder text of the nonce field should say "Automatically calculate."

I will review this with our team for whether we want to add a bounty before we do, but I think for some advanced users it could clearly be a nice thing to expose.

[SCBuergel]

I'll chip in if you Gitcoin this one!

[Meta: "to Gitcoin", verb, adding a bounty on the popular Gitcoin platform]

[bdresser]

handled in #7240