These avatar-related helper functions are used in pretty-text, which currently means we load the entire `discourse/lib/utilities` module into the mini-racer when running pretty-text on the server side. This stops us adding any logic or imports to discourse/lib/utilities which may depend on other `discourse/` namespace features.
This commit moves the avatar-related utils into a dedicated module in the `discourse-common` namespace, adds backwards-compatibility shims, and updates the pretty-text config accordingly.
