Inviting relaybot doesn't work if bot hasn't been encountered by telegram account

[tulir]

Users need to be encountered via username/group membership/etc so that the library would have an access_hash before the user can be interacted with (like invited to a group). This means that inviting the relaybot could cause an error if the user inviting the bot hasn't encountered it before. To solve this, the bridge should catch the error and search for the bot by username.