Add mechanism to automatically keep up with Omicron Dendrite and OPTE versions

[rcgoodfellow]

If the Maghemite dependencies for Dendrite and OPTE fall out of sync with Omicron, it can lead to iterop failures at runtime.

There are a number of ways to approach this. One option could be to have a dedicated branch for Omicron tracking and a reflector bot to keep common dependencies synchcornized with Omicron. I think the main branch should be able to run ahead of Omicron, and there should probably be a dependency job that keeps Maghemite mainline up to date with Dendrite and OPTE mainline.