Add IndexersHandler and its three sub-handlers, EditIndexerHandler, IndexerSettingsHandler and TestAllIndexersHandler, and dispatch to them from ReadarrHandler.
IndexersHandler::accepts ORs in all three children, so the parent and the children land together. Splitting would leave eighteen blocks accepted but inert, with Esc falling through to handle_clear_errors instead of popping.
TestAllIndexersHandler::accepts compares the block for equality rather than consulting an array, since TestAllIndexers appears in no block array.
