Support arbitrary tag on event to suggest not showing link previews on that event?

[turt2live]

Ideally used by bots who spit out enough context around the link to not need a preview. Clients could obviously ignore this, but it could also respect the hint. Example structure:

{
'msgtype': 'm.notice',
'text': 'Hello world! Here's a link to Google: https://google.ca',
'url_preview_hint': 'hide'
}

[turt2live]

Duplicate of #975 (github bot was slow)