Allow HTML in invitation message

[boonebgorges]

See https://wordpress.org/support/topic/any-plans-for-an-update/

The original suggestion is:

If the setting is set to where the member cannot change the body of the text, you could allow limited HTML.

The original issue here was not so much security, but that BP emails didn't support HTML. But BP emails have allowed HTML email for a number of years, and Invite Anyone has supported this for a couple of years now. See #143. So it should be possible to allow some limited HTML in this body, with the understanding that it will be stripped opt for plaintext emails.