phone nr - based registration and login

Hi,

I'm posting a feature request about this [again](https://github.com/vector-im/element-android/issues/2026), because my proposal appears to bounce from one repo to the other. The request is: to make a 1) registration and 2) login possible onto the Matrix ecosystem, with just a phone nr and SMS confirmation, like happens with Quicksy.im for XMPP, and Telegram, Signal, Whatsapp. In Quicksy.im a password can then be accessed in the User Account settings, so that it becomes possible to also use the account in other clients.

@ilmaisin [described it well](https://github.com/matrix-org/matrix-doc/issues/2740#issuecomment-694446187) by saying:
> Matrix indeed might benefit from a counterpart of [Quicksy](https://quicksy.im/). It would reduce the barrier of entry for nontechnical users. Currently, many people will likely either use a weak password or opt to not use Matrix at all.

@quotient-im mentioned [here](https://github.com/matrix-org/matrix-doc/issues/2740#issuecomment-705491943) that this is possible server-side, and the request should be laid with the client. So hereby I am making the feature request again with the client that i use, Element on Android.

Start by reading the linked prior issue 2026 and matrix-doc issue 2740 to understand the server-side and client-side boundaries. Then trace Element Android's registration and login entry points; done means supporting phone-number registration and login with SMS confirmation, plus access to a password in User Account settings.

Written by the indexing model from the issue text.