Add new field for custom per-app notifications to be pushed to the client.

See discussion in fdroidclient#195 (closed).

The goal is to let maintainers push arbitrary messages to the F-Droid client about an app, to notify them of important changes they should be aware of.

Field name: I don't know, "NotificationMessage" or something like that? @krt thoughts on the name for this field?

As far as I can gather from #195 (closed), this is non-controversial, has been wanted for a while, and it has the following requirements:

• Can be set for an app entry (not individual apks).
• Multi line text, allowing for URLs so maintainers can point to relevant info.
• For now it is only manually set. If in the future we want to hook it into the output of Drozer or other tools, that is fine, but it is not a requirement now.

@eighthave: Is this as simple as something like cb49f57c? If so, I'm happy to do it if you don't have time.