This includes changing the URL to #settings/preferences, with a
transparent redirect so that existing links, like the one from Welcome
Bot, continue to work.
Rename the 'Theme' section to 'Emoji'. Since there is no 'Theme' section
anymore, replace the 'theme-settings' classname with
'emoji-display-settings' in the 'settings_display.js' file.
The existing "Theme" selector dropdown is moved from the now the
'Emoji' section to the 'General' section.
These files are not Jinja2 templates, so there's no reason that they needed
to be inside `templates/zerver`. Moving them to the top level reflects their
importance and also makes it feel nicer to work on editing the help center content,
without it being unnecessary buried deep in the codebase.