App expiration tweaks

includes https://github.com/opentechinstitute/luci-commotion-apps/pull/58

to test:
0) flash two nodes
1) set `applications.settings.lifetime` to `60` in `/etc/config/applications` on both nodes
2) add application to node A. Node B should assimilate the advertisement. After 60 seconds have passed, make sure the advertisement is still on node B.
3) on node A, set `applications.settings.allowpermanent` to `0` in `/etc/config/applications`.
4) add app to node B. Node A should assimilate the advertisement. In the app's section in `/etc/config/applications` on node A, `lifetime` should be `0`, and there should be an `expiration` field set to 60 seconds after the node was added. After 60 seconds, the app should disappear from node A.
5) Now set `applications.settings.lifetime` to `120` and `applications.settings.allowpermanent` to `0` on node B. Add a new app to node B; it should still expire on node A after 60 seconds.
6) Set `applications.settings.lifetime` to `240` on node A. Add an app to node B. It should expire on A after 120 seconds.