Flag standard config as YAML comment

[arildm]

As a user, if I edit/upload a custom config, I want the standard config form to warn before saving, as it will overwrite my custom changes.

An issue is that we cannot really tell if the current config of a corpus is standard or custom.

Thus, the idea (by @MartinHammarstedt) is to add a magic comment in the config when saving from the standard form. We'll need to make sure that the comment remains throughout the CRUD workflow. It shouldn't be a problem for Sparv.

Another suggestion is to add a dedicated flag in the backend:

• Set/retrieve value for creation of custom config mink-backend#114