Disable auto-upload by default

[smichel17]

Use case

I was planning on discussing this eventually anyway, but it's also timely now: in #3466 (comment), @westnordost wrote,

Now, one can argue that the precise/coarse location of the user can be determined by

• his current contribution history (especially if he has auto-sync on)

• When we used overpass, auto-upload was very important, because some quests (house number) would not appear until after a pre-requisite quest (building type) had been uploaded. However, with offline data storage, this is no longer a requirement. Actually, it's the other way: some functionality is only available when auto-upload is off (undo of certain actions, like leaving notes).
• Auto-upload has a large impact on battery usage (Adapt app behavior when power saving mode is enabled #3251).
• And it's probably the biggest privacy concern in the app.

Proposed Solution

Change the default setting to upload on wifi only.

Risks

Some people may not realize they need to upload now. Maybe there could be a prompt, similar to when you are not logged in yet, if you reach a certain number of changes waiting to be uploaded or if you have changes which are older than a certain age. It would tell (or require?) the user to upload, and let them know about the auto-upload setting (with a warning about battery usage).