Fix default builtin group being associated twice with imported users

[wtfzambo]

Fixes #96.

Also adds a progress bar to the operation and some minor polishing.

[susodapop]

The code looks good. Since we don't have progress meters for any other operations, I would rather not introduce the complexity here. It might lead users to believe that the other commands are somehow broken.

Can we split this into a separate PR and implement it for all the commands?

[wtfzambo]

The only reason I made the progress bar is because there's not API to delete members in batch for any given group. So I had to use the api/groups/dest_id/members/member_id endpoint and make a DELETE request for every member_id found in the group.

I didn't want to show an operation for each row like it happens for every other command, because it didn't make sense in this case, but on the other hand, if one has many members (for example, my instance has 83) it can take a while, thus I added the progress bar to show the user that the script is not stuck and it's doing its job.

If you want to add it for every command, what should we do with the current print operations? In terms of user experience I don't see the progress bar being compatible with them, since for each id of something being exported, a new line is printed.

[wtfzambo]

@susodapop
I could maybe try implementing something like this, if it's to your liking (but without using external packages). Although for the "Preparing builtin group {dest_id} to restore original members associations..." operation I still would skip printing each DELETE operation. Up to suggestions tbh.

[susodapop]

You convinced me 👍

Tested this against the latest redash V10 image (10.0.0 (9c928bd1)) and all works.