api: Rename stream to channel in events

[Khader-1]

Fixes parts of #631

[rajveermalviya]

LGTM, moving on to mentor review from @kenclary

[Khader-1]

Thanks @rajveermalviya for the review! I'm going to skip mentor review and maintainer review for this one.
Refer to Greg's comment on #801

let's use an abbreviated process where we go straight from buddy review to integration review, skipping mentor review and maintainer review.

[gnprice]

Thanks! These changes look good — merging.

I squashed the four commits into one, because they're all closely related and the size of the diff remained quite manageable; from git log --stat:

 lib/api/model/events.dart           | 34 +++++++++++++++++-----------------
 lib/api/model/events.g.dart         | 12 ++++++------
 lib/model/channel.dart              |  6 +++---
 lib/model/store.dart                |  4 ++--
 test/model/test_store.dart          |  2 +-
 test/widgets/message_list_test.dart |  2 +-
 6 files changed, 30 insertions(+), 30 deletions(-)