[ROSADMIN] Add Mattermost plugin for admin panel

[binarymaster]

• Shows user sessions in Mattermost with detailed information
• Allows to revoke specific or all user sessions

Requires a privileged token to work on all users.

[ColinFinck]

Thanks for your efforts Stanislav and sorry for the long delay from my side!
Your PR looks good, just a few little things that need changing.

[binarymaster]

Thanks for the review, I have done suggested changes in a new commit.

[ColinFinck]

LGTM. I lack a suitable testing environment right now, but after reviewing the code in-depth, it should be fine to merge this particular PR and fix potentially outstanding production bugs afterwards.

Thanks for your efforts and sorry that the review took so long!