Too easy to create multiple unlinked accounts

[jidanni]

Warn users on https://mapwarper.net/u/sign_up that of the four methods,

• Email
• OpenStreetMap
• GitHub
• Facebook

they should pick one, and stick with it, to avoid creating multiple unlinked accounts.

For instance, if #233 happens, then like https://mapwarper.net/u/sign_up says,

Didn't receive confirmation instructions? Alternatively, sign up with:

Then they probably will choose create via OSM. Then later, when they get the email, and they confirm it, voila, 2nd unlinked account is created. The only way to clean up is to Cancel that account.