Add reply previews to status composer

Reasoning

When replying to someone on Mastodon, it's really really easy to forget what the original post was - especially since the higher character limits compared to Twitter. Since Tokodon's composer takes up a whole page right now, the problem is exacerbated because it involves cut & pasting your draft (since we also lack a draft system) and backing out of the composer manually. A preview of the post you're replying to will go a long way!

Changes Needed

A lot of the needed properties aren't actually a part of Post, but instead exposed through AbstractTimelineModel. A big chunk of this MR is simply refactoring it out so we can use them on the QML side without the need for a model. Other changes touch other parts of StatusDelegate components, and trying to separate them from the main delegate they were previously used in. It's not perfect, but it's a start.

Screenshot

To-do

• Test against posts of all types (ones with links, ones with multiple attachments, videos? etc)
• Refactor the replyTo signal because we might only need to pass the post property now.
• Fix the copyright header in PostContent.qml