We found that non-templated broadcast messages weren’t having their
content normalised before saving into an event.
This means that stuff like `\r\n` and curly quotes were being passed
through to the CBC proxy.
This commit firstly changes templated events to use
`str(BroadcastMessageTemplate)` to normalise the content, because it’s
non-obvious that calling
`BroadcastMessageTemplate.content_with_placeholders_filled_in` also
normalises content.
Then it changes the non-templated route to also call
`str(BroadcastMessageTemplate)`, where previously it was passing the
content straight through.
0a3be6a662