Add an option to completely prevent upload of a layer : e.g. "never" to upload=true/false

[jaakkoh]

The most typical need for this extension might be training totally new people to the world of OSM, specifically using JOSM. There is commonly e.g. an example.osm file used to illustrate elements in OSM and how they show up & are manipulated in JOSM. The issue has been discussed at ​https://github.com/hotosm/learnosm/issues/335 . And the (currently last) comment from June 1, 2015.

Currently the most that can be done is to

• use the upload=false and
• then tell those that are trained not to upload their edited data -- and definitely not to "bypass" the prompt that warns not to upload the data (if someone does try to upload. And finally to
• check the area where the example data is placed to see if anyone has ended uploading regardless of the above.

Experienced users and especially those that are more computer savvy than some might think that having upload=false is sufficient. But it's not. And the answer is not to put more emphasis to stressing the need to not upload when the upload tells not to.

So, simply said, there really should be an upload=never option, or something alike.

[jaakkoh]

(comment moved to the ticket itself)