Feature: Merge several Slack exports

[svenssonaxel]

There are several scenarios where it'd be nice to transform several Slack exports to one MM import:

• One is unable to obtain a Slack corporate export, but the number of users are sufficiently small that it's possible to obtain one export from each user's perspective that includes direct messages.
• One would like to migrate several Slack workspaces to one Mattermost team.
• One happens to have several Slack exports with overlapping time periods.

It's possible that some of the above scenarios, at least some of the time, can be solved by transforming and importing each export separately, but there is at least two reasons why this might be undesirable:

• Transforming and importing a large number of Slack exports one at a time can be tedious, since one first has to upload it, then obtain an id to run the import job and check the result.
• Some Slack exports can have some user information missing that is present in other exports, such as email addresses for users. Currently, if one were to transform/import such an export, mmetl would make up an email address for this user. Then, when trying to transform/import another export that contains the correct email address, the mismatched email address would cause the import to fail.

[huyz]

For this issue:

Some Slack exports can have some user information missing that is present in other exports, such as email addresses for users. Currently, if one were to transform/import such an export, mmetl would make up an email address for this user. Then, when trying to transform/import another export that contains the correct email address, the mismatched email address would cause the import to fail.

I wonder if this could help "pre-process" the exports before mmetl tries to import: https://github.com/grundleborg/slack-advanced-exporter