fix(oauth): increase the assertion lifetime to avoid clock skew issues

Fixes #1649.

@ckarlof r?
Actually, here's a longer list of places where we're making up some times:
```sh
$ grep -Ir "1000 *" . --exclude-dir="node_modules" --exclude-dir=".tmp" --exclude-dir="app/bower_components" --exclude-dir="dist"  | grep -v ".svg:" | grep -v "bower_components" | grep -v app/scripts/vendor/

./app/scripts/lib/assertion.js:  var CERT_DURATION_MS = 1000 * 60 * 60 * 6; // 6hrs
./app/scripts/lib/assertion.js:  var ASSERTION_DURATION_MS = 1000 * 60 * 5; // 5mins
./app/scripts/lib/channels/fx-desktop.js:  var DEFAULT_SEND_TIMEOUT_LENGTH_MS = 1000;
./app/scripts/lib/metrics.js:  var TEN_MINS_MS = 10 * 60 * 1000;
./app/scripts/views/form.js:    LONGER_THAN_EXPECTED: 10000, // 10 seconds
./app/scripts/views/mixins/resend-mixin.js:  var SHOW_RESEND_IN_MS = 5 * 60 * 1000; // 5 minutes.
./app/tests/spec/views/confirm.js:            }, view.VERIFICATION_POLL_IN_MS + 1000);
./server/bin/csp-violation-server.js:      when: (new Date()).getTime() / 1000,
./server/lib/configuration.js:    default: 10 * 60 * 1000 // 10 minutes
./tests/functional/firefox/same_account.js:        .sleep(1000)
./tests/functional/lib/helpers.js:    }, [ selector ], 10000);
./tests/lib/restmail.js:          }, 1000);
./tests/server/configuration.js:    var dfd = this.async(10000);
./tests/server/metrics-collector-stderr.js:    var dfd = this.async(1000);
./tests/server/proxy.js:    var dfd = this.async(1000);
./tests/server/proxy.js:    var dfd = this.async(1000);
./tests/server/proxy.js:    var dfd = this.async(1000);
./tests/server/proxy.js:    var dfd = this.async(1000);
./tests/server/proxy.js:    var dfd = this.async(1000);
./tests/server/proxy.js:    var dfd = this.async(1000);
./tests/server/proxy.js:    var dfd = this.async(1000);
./tests/server/templates.js:      var dfd = this.async(1000);
```

Plus the few places where we did `* 1000` instead of `1000 *`:
```sh
$ grep -Ir "* 1000" . --exclude-dir="node_modules" --exclude-dir=".tmp" --exclude-dir="app/bower_components" --exclude-dir="dist"  | grep -v ".svg:" | grep -v "bower_components" | grep -v app/scripts/vendor/

./app/scripts/lib/metrics.js:  var TEN_MINS_MS = 10 * 60 * 1000;
./app/scripts/views/mixins/resend-mixin.js:  var SHOW_RESEND_IN_MS = 5 * 60 * 1000; // 5 minutes.
./server/lib/configuration.js:    default: 10 * 60 * 1000 // 10 minutes
```