1.3.0-beta2: form WSOD fix + config backfill hook + branch consolidation

Two bug fixes from the 1.3.0-beta1 production-soak handback. The
soak halted at Day 0 because neither bug let any traffic reach the
install.

Fix 1 (critical, form WSOD): the ConfigTarget closures for
private_cache.contexts and vary_cookies declared strict array
parameter types. Config::get() returns NULL when a key is absent;
the strict typing threw a TypeError inside the closure and took
out the entire settings page. Both closures now declare ?array and
fall back to an empty list.

Fix 2 (high, config backfill): new lscache_update_8103 walks the
install YAML and writes any missing default keys to active config
without overwriting operator changes. Generic implementation that
backfills any missing key from config/install/lscache.settings.yml,
so future schema additions pick up the same behaviour automatically.

Branch consolidation: from 1.3.0-beta2 forward, all LSCache
development happens on 1.3.x. The 1.1.x and 1.2.x branches are
frozen at their respective beta1 tags. Operators previously on
those should bump to 1.3.0-beta2 to pick up the fixes; 1.3.x's
cumulative branch model means every prior feature is preserved.