Only create preview hst config branch if it does not yet exist

If there is not yet a preview configuration of a channel, this preview config is added on first access when opening a channel in CM. If there are branches for the channel, and those branches do not have a preview, also the preview for the branches are created.

In general, when working with the CM, it can never happen that there is no -preview config for master but the branches do have a preview. However, when the preview of master is removed manually, it can of course happen.

If the above is the case, then when opening a channel, an exception is thrown because it is tried to add a preview configuration for a branch that already has the preview