Add a USER_URL_PREFIX configuration option

[sangaline]

I think that it's a common use case that people want to put all of the user urls under another prefix other than '/user' (e.g. '/admin', '/'). Might it make sense to add a new config option that lets people change all of them at once instead of having to specify all 12 individually? The proposed implementation in this pull request would be backwards compatible while adding a small convenience for those wanting to specify a different url prefix.

[and-semakin]

Hi @sangaline! Thank you for your contribution! It looks super useful. Could you please rebase your branch over current master branch to resolve conflicts?