Since _ls is now deprecated this is removed in favour of L1, I've also taken the oportunity to replace QStringLiteral and QLatin1String with their shortened form while we're at it.
There are also a few instances where the string literal type has been switch, the general rule being to use the one that matches the function type or value being compared to avoid conversions.
Refactor `LinkPreviewer` to take an event and put the functions for getting the link in the class itself. This means the functions in `EventHandler` are no longer required.
This mr also sets up `LinkPreviewer` so that it is automatically updated when an event is edited. This includes changing the link if edited, and it can handle a message having a previous link removed or a one added when one didn't exist before.
Also adds test suite.
