- add config key ALLGO_WEBAPP_DEFAULT_MEMORY_LIMIT_MB to set the
  default memory limit for newly created webapps

- add memory limit field in WebappUpdate

- display memory_limit in megabytes in the UI (through the
  memory_limit_mb form field)

- disable the memory_limit_mb widget and ignored the posted value
  when user is not superuser

- forbid negative value in the model

- make the memory limit field required in the forms