Replies: Implement sending on mobile

[manuroe]

Following #148 where we display reply-to messages, Riot mobile apps need to be able to send a reply.

On Riot mobile apps, this is possible to select a message. As a first cut, we will use this behaviour to implement the reply feature without any change on the UI. That is:

If a user types a message while a message is selected, the typed message will be considered as a reply to the selected message.

We will also add a reply option to the message context menu but it will have not much more effect.

The implementation of the sent reply must conform to the specification at https://docs.google.com/document/d/1BPd4lBrooZrWe_3s_lHw_e-Dydvc7bXbm02_sV2k6Sc/edit. Specifically, we need to manage plain text fallback.