Doesn't immediately replace usernames after switching channels / pages

This is a pretty bad bug; apparently `waitForKeyElements` doesn't always trigger when switching channels or pages. I'm not sure what the best way to fix this is, short of just setting a timer to call `replaceAll()` every few seconds or something. Suggestions welcome.

## Workaround

Open up the mappings dialog and hit the `Save` button.

Start by reading the implementation and call sites for `waitForKeyElements` and `replaceAll`, then reproduce the issue by switching channels or pages. Done means usernames are replaced without opening the mappings dialog and pressing Save; no test file is mentioned in the issue.

Written by the indexing model from the issue text.