feat(alertmanager): create config on first user creation

feat(msteams): add migration for msteams

feat(config): add Yamlv2Unmarshaller func

feat(msteams): add migration for msteams

test(matcher): add test cases for matcher

feat(msteams): add migration for msteams

chore: add new user preferences for welcome checklist (#7239)

fix(alertmanager): fix ReceiverNamesFromRuleID logic

fix(alertmanager): let defaults kick in for the receiver

fix(legacy): fix TestAlert