Provide configuration to enable/disable ability to cancel jobs in Web Frontend

The Web API can be configured to prevent job submission by setting jobmanager.web.submit.enable to false in the Flink YAML configuration file:

# Flag to specify whether job submission is enabled from the web-based
# runtime monitor. Uncomment to disable.

jobmanager.web.submit.enable: false

Users can still cancel running jobs. We would like to be able to offer the Web Frontend to teams for monitoring of jobs, but since this is shared infrastructure we do not want users to be able to run or cancel jobs through this interface.

It is proposed that the ability to cancel jobs is also configurable in the Flink yaml configuration file.