NUX improvement and simplification

Main changes:
* welcome screen replaced by the sign in screen
* create new account in one step
* no tutorial
* hide self-hosted url by default (tap "add self-hosted" to make it visible)
* email domain name autocorrection
* styling (new icons, change input styles, remove some dialogs)
* refactoring (create account / setup blogs)

Note: needs more cleaning, but it's an app-wide task see #328