CLEAN_INSTALL wipes /opt/shlink-web-client on every `update`, deleting the
user's servers.json (and its API key) with no backup, unlike the main
/opt/shlink app which already backs up/restores .env and data around its
CLEAN_INSTALL. This makes the Web Client lose its configured server(s) on
every update.
Reuse the existing create_backup/restore_backup helpers around the
web-client's CLEAN_INSTALL, mirroring the pattern already used for
/opt/shlink.
Replaces the misc/build.func bootstrap with the two-root core loader.
No other change: the engine contract (var_*, update_script, start,
build_container, description) is identical on both engines.
versitygw.sh additionally gains a trailing newline; it lacked one.
With this batch every ct/ script is on the core engine.
