Colorize nicknames in message content

**Is your suggestion related to a problem? Please describe.**
In a busy room, when people are referring to each other for directed conversation, it is hard to tell who is talking to who without colorized nicknames at a glance. It's not easy to tell whether they are talking about something or whether they're referring to a person in the chatroom:


The top half of the picture shows a conversation that doesn't colorize the nick Rukus inside Element Web in the message content
The bottom half of the picture shows a irc client which colorizes the nickname Rukus so its easy to see they are talking to a person in the room, instead of something called Rukus

**Describe the solution you'd like**
Element Web should imitate the irc client and colorize nicknames within messages. This way, immediate context is given visually, and when the color of the nickname inside the message matches a person's nickname color when they post, it is easy to correlate who talked when and what they said and who is mentioned and what they said. Without this, it's easy to lose this information, and assume Rukus might be an object (like a popular networking equipment). What is the point of colorizing nicknames but not mentions in messages? Seems like not colorizing nicknames in messages is a half coordination instead of full colorized implementation that can be followed from nickname to message content and vice versa.

**Describe alternatives you've considered**
The IRC client way is one alternative, another would be to make a pill out of the mention.

The issue names no files, tests, or entry points. Start by locating Element Web's existing nickname-coloring and message-rendering paths, then compare the requested behavior with the screenshot and alternatives described. Done means nicknames mentioned in message content receive distinguishable, consistent coloring or an agreed mention-pill treatment.

Written by the indexing model from the issue text.