* introduce ExtraConfigsCrypto
* delete getLastConfiguration and move all code into method Get
* update legacy store to encrypt ExtraConfigs same way we do in MultiOrgAlertmanager
* update legacy store init
* delete PersistConfig
* remove unnecessary unmarshalling
* add better error
* add tests for legacy store
* add tests that cover extra config validation
* add integration test for conflicting routes
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Signed-off-by: Yuri Tseretyan <yuriy.tseretyan@grafana.com>