This is to break the bigger functions into smaller
ones and hence, makes unit testing easier.
Also includes renaming the stream's topic name to 'from slack'.
Previously we had a problem of id clashes while importing converted
slack data into an existing zulip instance with realms which are actively
populating the database.
This counts the total objects to be imported and does a db transaction
to increase the SEQUENCE number for that table by that number,
and hence allocates a range of ids for the to be converted slack data
objects.
* For strikethrough formatting: Slack's '~strike~' to Zulip's '~~strike~~'.
* For bold formatting: Slack's '*bold*' to Zulip's '**bold**'.
* For italic formatting: Slack's '_italic_' to Zulip's '*italic*'.
* For mentioning formatting: Slack's <@slack_id|short_name> to Zulip's @**full_name**.
* Checking links.
Currently the zip file is extracted to the root of
zulip directory no matter where the the zip file.
The extracted data is not useful after running the command
which pollutes the zulip directory. It make more sense to
extract it to the same directory of zip file especially
when the zip file gets downloaded to /temp like in the tests.
We no longer have a special UI setting and model
field ("emoji_alt_code") for saying users want text-only
emojis. We now instead make "text" be a fifth choice
for "emojiset".
Fixes#7406
