code cleanup / fixes

[jaroslawp]

• removed unused references to team_id
• removed unneeded workaround ('joined channels')
• merged same code duplicates in single functions
• started using MattermostChannel / Mattermostuser types instead of strings.
• removed unneeded channel name separator (---)

[jaroslawp]

Turns out I made quite a lot of copy&paste programming last month, this should clean it up.

[jaroslawp]

I experience some crashes with this branch, lets close it and rework later