Add a config setting to "enable local mode"

[James Simpson]

https://docs.mattermost.com/configure/configuration-settings.html#enable-local-mode

This would be useful for operations such as upgrading the license key (https://docs.mattermost.com/upgrade/installing-license-key.html), where we're currently met with:

root@mattermost-677d4ddf4f-qzgcd:/mattermost# /mattermost/bin/mmctl license upload /secrets/licence.txt
Error: cannot read user credentials, maybe you need to use login first: stat /root/.config/mmctl/config: no such file or directory

root@mattermost-677d4ddf4f-qzgcd:/mattermost# /mattermost/bin/mmctl --local license upload /secrets/licence.txt
Error: socket file "/var/tmp/mattermost_local.socket" doesn't exists, please check the server configuration for local mode