Add chat mentions and a notifications panel (#3121)

### Todo

* Displaying notifications
    * [x] show them in panel
    * [x] indicate read/unread status
    * [x] allow requesting more by scrolling down
    * [ ] style the panel
    * [x] style the status bar icon
* Chat mentions
    * [x] highlight mentions when editing message
    * [x] persist mentions
    * [x] highlight mentions when rendering saved messages
* Creating notifications
    * [x] contact request received
    * [x] contact request accepted
    * [x] channel invitation received
    * [x] mentioned in a chat message
* [x] Indicate responses to notifications
* Mark notifications as read
* [x] when viewing a contact request acceptance in the notification
panel
    * [x] responding to contact invite
    * [x] responding to channel invite
    * [x] viewing a channel message mention
* [x] Replace previous notifications with auto-dismissing,
non-interactive toasts

### Release Notes:

- Added a notification panel, which displays notifications about contact
requests and channel invitations.
- Added the ability to `@`-mention users in the chat, so that they will
be notified of your message.