Disallow adding/editing localfs integration from ui

[nemunaire]

To follow up #240 (comment), this addresses the possibility of adding or editing localfs directly from the UI.

Instead it shows the content to add/edit in .userprofile:

To be able to pass error directly from Go code, I added a convention that errors reported as JSON should be passed to the UI as:

c.AbortWithStatusJSON(http.ErrorStatus, gin.H{"error": err.Error()})

[ddvk]

we could add a config variable for allowed root paths for localfs, which would mitigate the issue.