We (lexically) remove "subject" from the conversion code. The
`build_message` helper calls `set_topic_name` under the hood,
so things still have "subject" in the JSON.
There was good code coverage on `build_message`.
We now use subscriber_map for building UserMessage
rows in Slack/Gitter conversions.
This is mostly designed to simplify the code, rather
than having to scan the entire subscribers for each
message.
I am guessing this will improve performance for most
conversions. We sort small lists on every message,
in order to be deterministic, but the sorting cost
is probably more than offset by avoiding the O(N)
scans across all subscriptions. Also, it's probably
negligible in the grand scheme of things, compared
to JSON parsing, file I/O, etc.
This commits also fixes some typos with mentioned_users_id ->
mentioned_user_ids and cleans up a test a bit as well.
The missing fields are checked by `full_clean()` method.
The datetime field errors are ignored as they are fixed
in the `import_realm` script. The field that are
allowed to be null are not included while building
this object.
