Text / yaml editor for config files

[manthey]

It would be handy to adjust configuration if it could be edited in the girder interface. This would, for text, json, and yaml file types, expose an edit button that would open a simple text editor. On submit, it would validate the yaml or json as appropriate by mime type and then replace the contents of the file with the new value. It would only work for smallish files.

We'd want to have sensible error messages for invalid yaml/json.

[manthey]

See girder/large_image#969