Update for 5.4

- Add `SERVERIGNORE_ENABLED` to TOC SavedVariables tag (allowing the
enabled state to be saved correctly)
- Replace individual event listings in `Add/RemoveFilters` functions
with single table of events to filter
- Add `printf` function for slash command
- Improve detection of own server in `add` and `remove` commands
- Replace manual concatenation of server names in `list` command with a
temporary array and `table.concat`
- Rewrite ADDON_LOADED code to set `SERVERIGNORE_ENABLED` default using
an explicit `if` instead of `and`/`or`.