Change "DM" and "Group Messages" icons to be readable by mere mortals

**Is your feature request related to a problem? Please describe.**
Currently the icons at the bottom of the screen for 1:1 chats vs. group rooms includes:
1. a speech bubble for 1:1 DMs
2. a circle with `#` inside

Users who are unfamiliar with IRC or Slack or other more technically-focused conversation platforms have no recognition of the relevance of the `#` symbol equating to a chatroom, and visually these two icons do a poor job of demonstrating the differences between these two kinds of conversation.

**Describe the solution you'd like**
The solution would be to change these icons to represent:
1. a single person for 1:1 DM rooms
2. a group of three people for multi-participant rooms (see https://static.thenounproject.com/png/446003-200.png for a simple representation of this)

**Describe alternatives you've considered**
A more advanced function might be to allow users to configure which set of icons they want within the application, although this seems unnecessary if the icons are in-line with the design style of the application and _do a reasonable job of representing what they are for_