Update database with feature level etc. on each register

[gnprice]

On the Account class, corresponding to a row in our local database, we have several fields for values that may in reality change over time: most notably zulipFeatureLevel, also zulipVersion and zulipMergeBase.

Currently we take the values of those fields when first logging in, and don't ever update them later. We should do so. In particular when we have compatibility logic that conditions on zulipFeatureLevel, that will allow that logic to correctly use the server's current version rather than the version it had when we first logged in (perhaps months ago). Discussion here:
https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/zulipFeatureLevel.20out.20of.20sync/near/1708193

Specifically this issue is for updating these values using the registerQueue response. We should also do so on restart events; that's tracked as part of #135.

I have a draft branch for this which I wrote one morning a few months ago and then didn't finish. (That branch handled restart events, which aren't so much a priority in themselves, but most of the work involved was for the ability to update account records in the database at all.)

[gnprice]

Another effect of this is that it will serve as an early small trial run of the sort of code changes we'll need for #477, because it'll involve writing changes into the database in tandem with making them in our Dart data structures.

[gnprice]

I have a draft branch for this which I wrote one morning a few months ago and then didn't finish

I've just sent #526, built around one commit from that branch but then with 14 other commits of followups it led to.

I've also rebased the branch as a whole and updated the rest of it a bit, and have it still as a draft branch.