admin filter for user listing

[JacksonChen666]

the admin filter is a tri-state, meaning it can filter for admins, filter for non-admins, and not filter at all.

fixes #122

NOTE: It is a draft PR because I am looking for a cleaner solution of "the user said no based on this flag here" rather than the current manual solution of checking. If that's not possible, then this is pretty much complete. found a cleaner solution

[JacksonChen666]

merging because i've already tested and it works

[a-0-dev]

This was a breaking change for us (using synadm for scripting). The user_list_paginate function uses user_list, but the call of user_list was not updated to include some setting for the admin filter, so calling user_list_paginate inherently fails.

Correct me if I'm wrong, but to me it looks like this may be the problem, so potenially just fixing api.py:471 or extending user_list_paginate's signature with the admin filter would be sufficient. If doing the latter, please consider putting a default value for the admin filter so there's no permanent breaking change for others using the function :)

Thanks for your work on synadm, we highly appreciate it!

[JacksonChen666]

@a-0-dev Thank you for your report, please do create issues in the issue tracker the next time you spot an issue. The issue you found has been made at #127.