Delay authentication timeout on scope updates

Fix two issues with the authentication timeouts.

- Fix #1284, the regression causing resets to always occur, introduced
  by e4f32116fe5f266e2030a666f5cc912b6b3df308
- Flash a message when the reset happens. Close #1280
- Increased the timeout to five minutes, as per suggestion by @dwhly
Merge pull request #1285 from hypothesis/1280-auth-form-timeout

Delay authentication timeout on scope updates