SubmissionsController: Remove outdated `ActionController::Live` mixin
SubmissionsController: Remove outdated `ActionController::Live` mixin

The mixin was previously used for Server-Sent-Events, which were removed from CodeOcean a long time ago.
After the mixin is removed, we can also fix the cookie send mechanism (this was erroneous with the mixin).

openHPI/codeocean,openHPI/codeocean,openHPI/codeocean,openHPI/codeocean,openHPI/codeocean,openHPI/codeocean