Chrysalis should be able to auto-update from release to snapshot

[algernon]

There are times - especially now, when Chrysalis is moving fairly fast - when one would like to try the latest snapshot. Currently, that requires going to the release page, finding the latest pre-release, and downloading that. It would be nice if Chrysalis' auto-update configuration would allow setting the channel for auto-updates, so we could just switch to snapshots, and Chrysalis would auto-update to the latest one.

[algernon]

I plan to change the auto-update UI a little, and instead of an on-off toggle, we'd be able to select the channel used for auto-updates:

Select a channel for automatic updates          [  Automatic   ⌄]
                                                |  Release      |
                                                |  Snapshot     |
                                                |  None         |

Automatic would be the current behaviour: either Release or Snapshot, depending on the current version. Release would pull from the release channel, snapshots would consider snapshots as valid candidates too, and None would turn automatic updates off.

We'd have the same for firmware updates, too.

Chrysalis updates would continue to default to "None", but if previously they were enabled, that setting would be migrated to "Automatic". Firmware updates would continue to default to "Automatic", and similarly to the app channel, the former bool setting would be migrated to match the same behaviour within the new options.