DRY up protobuf declarations and move to a slightly briefer naming
convention.
Also dropped some ArrayBuffer -> string conversions as
ProtoBuf.js handles ArrayBuffers just fine, and in fact, more
efficiently than strings.
Finally, dropped the btoa() wrappers, because that incurs an extra
string -> string conversion before the protobuf's internal string ->
array buffer conversion. In lieu of btoa, we can simply pass in the
optional string encoding argument to the protobuf's decode method,
which in these cases should be 'binary'.
Related: #17
Each conversation views now manages its own separate elements
rather than all binding to a shared #conversation element, and
similarly for message composition ui.
Also includes the beginnings of group creation UI (not working yet),
featuring bootstrap-tagsinput field for entering group recipients
