Silently drop invites determined to be spam

[H-Shay]

Following on from matrix-org/mjolnir#300, rejecting spam invites can just precipitate more abuse, as it can alert attackers that their names have been added to a ban list. This could be addressed by silently dropping spam invites, rather than rejecting them with an error. For example if the spam check here
https://github.com/element-hq/synapse/blob/568051c0f07393b786b9d813a1db53dd332c9fc2/synapse/handlers/room_member.py#L900-914 determines that the invite is spam, return early with a 200 rather than raising an 403 error.
This could either be implemented as-is or could be a config option to give people the ability to choose to silently ignore spam invites.