room-history: Implement mention of users in the message entry

Start typing @ and a popover will show with suggestions of room members sorted by activity, then:

• Select the wanted user by clicking it or moving with arrow keys and activating with the Enter or Tab keys.
• Or hide the popover by pressing the Esc key.

As can be seen somehow in the screenshot, it has the same issue as Entry for user lookup in invite dialog is super... (#1001), where the entry grows too much when we insert a pill.

Part of Implement tab-completion popover (#766 - closed) (We should also add mentioning rooms before closing it).

Fixes Pills aren't aligned properly with text in Text... (#880).