Panic: Failed to create index pq: coluna "teamid" does not exist

Seems like all trouble with missing columns and Bad Gateway responses from Nginx had to do with skipped versions. Here ist how we got it solved:

• download all major mattermost releases that you have skipped. I had upgraded from Gitlab 8.0 to 8.4 which had skipped the following mattermost versions:
  -- 1.0.0
  -- 1.1.0
  -- 1.2.1
  -- 1.3.0
• download each version and extract them in to seperate folders
• stop mattermost (the correct command on my system was gitlab-ctl stop mattermost)
• go to /opt/gitlab/embedded/bin/mattermost and make a backup copy from the file mattermost
• replace it with the file platform from the bin folder of mattermost 1.0.0
• rename the platform file to mattermost
• start mattermost by gitlab-ctl start mattermost

The GUI won't work, but you may trace in your logs that mattermost started and updated your database. Repeat the replacing of the mattermost file with the platform binary of each mattermost version. Last but not least, place your latest version of mattermost in gitlab.