This makes sure the AngleSharp owner document is only created once per application lifecycle, and replaces all async calls with their synchronous counterparts (since the input is already loaded in memory, using async for this just creates overhead)
Inline media can be created by:
1. Attach media to note as usual
2. Copy media URL (public one, for remote instances)
3. Use the new $[media url ] MFM extension to place it wherever you
wish. (The trailing space is necessary as the parser currently
treats the closing ] as a part of the URL)
The Iceshrimp frontend may make this easier later on (by having a
"copy inline MFM" button on attachments, maybe?)
Federates as <img>, <video>, <audio>, or <a download> HTML tags
depending on the media type for interoperability. (<a download> is
not handled for incoming media yet).
The media will also be present in the attachments field, both as a
fallback for instance software that do not support inline media,
but also for MFM federation to discover which media it is allowed to
embed (and metadata like alt text and sensitive-ness). This is not
required for remote instances sending inline media, as it will be
extracted out from the HTML.
The Iceshrimp frontend does not render inline media yet. That is
blocked on #67.
