Fixes #215

Displays the warning after a successful login with "remember me" checked (or when you tick the "remember me" in settings, if you haven't already agreed to the risk).

The warning has two buttons.

1. "OK, I understand" - goes ahead and remembers the user. Also sets a flag in localStorage to say that they have accepted the risk.
2. "Forget it, forget me" - does *not* set "remember me" and unpicks the checkbox in the case of settings
"We suggest using caution" rather than "We suggest taking
caution".  (Sticking with idiom...)
Merge pull request #225 from SpiderOak/issue-215-warning-will-robinson

Fixes #215