Switch off task management temporarily

[tusmester]

We need a master switch for registering or executing tasks. Note that this may prevent some features from working correctly, because there may be missing tasks when we switch the feature back on.

The main use case was to switch off preview generation in a subtree while importing to prevent overloading the servers, but there is another issue for that: #1519.

Workaround: removing the task management url from config.

This a general task management switch, not only for preview generation, but for all types of tasks.

Questions

• is this a repository or a task management service switch? (tasks may be registered, but not executed?)
  • if it is a repo switch: setting or configuration switch?