Standardise email formatting

[matthewhilton]

Currently the formatting of emails is all over the place:

• Fields such as confirmationmessage are editor form elements (hence are html), but the rest of the email related fields are just plaintext with no format.
• Some functions use facetoface_send_notice but some don't leading to inconsistencies.
• There is multiple layers/combinations of format_text, nl2br, html_to_text being applied that tends to mangle the output of any replacements

[danmarsden]

Thanks @matthewhilton this has annoyed me for a while too so really nice to see it getting a tidy up!