Publish to Mastodon functionality

[lupinia]

As a content creator, I want to be able to post to my Mastodon account directly from my own website. I need to look into the ActivityPub API/protocol more, because I suspect it can open a lot more opportunities than I'm currently imagining. But, for now, I want the following functionality:

• Draft the body of a post from any page on the site (mainly DeerTrees content and directory listings), which will be sent along with a URL to the page I'm currently viewing.
• Include a CW field in the post editor.
• If the page is a Sunset image, embed the image in the post as an attachment, along with its corresponding alt text.
• If the page is a DeerTrees category with featured images, embed a random selection of up to four of the feature images in the post. If it has images but they're not featured, embed a random selection of up to four regular images in the post. Include an override for this. And include alt text for all images.
• If the page is a DeerBooks page with images associated with it (not currently a thing, but it'd be nice to add eventually), embed one of those images. There are too many hypotheticals and undefined features to really flesh this one out too much.
• If the page is DeerTrees leaf content, and there are public external platform links defined, include those as well (with an override).
• By default, submitting a post queues it to be posted later, with a per-day post rate and a time range during which posts can be sent. The async post submission maintenance task (run as a cron job) selects a random time during these hours to send a post.
• I also want a "post now" option, which bumps the post to the top of the queue and sends it at the next cron run without delay, and a "schedule" option that sends it at a specific date and time instead of randomly selecting within a range.
• Include an option to draft an arbitrary post with no specific link, because if I'm going to the trouble of coding what is effectively a replacement for Buffer, I might as well go all the way.
• If the page is DeerTrees leaf content, automatically add the resulting URL to the external links after posting.
• Include a "reply to" option, where I can post a link to a toot to reply to, either my own or someone else's.

[lupinia]

Making a note here, because I don't have a better place to put this thought at the moment: In theory, the work I did to implement OpenGraph tags in #61 should negate the need for embedding the images into a post as attachments, because that's kinda the whole point. However, unless Mastodon gets a whole lot better and a whole lot more reliable at showing the link previews, redundant image attachments will probably still be necessary. Which sucks, because unlike Twitter, Mastodon storage is at the direct expense of instance admins, and I don't really want to consume more of it than I have to (even if my own personal usage is an insignificant drop in the bucket relative to the size of my current primary instance, it's about the principle of the matter!). Additionally, as of this writing, Mastodon doesn't include the alt text for link preview images, even though the OpenGraph specs support this; a ticket tracking that is here: https://github.com/mastodon/mastodon/issues/18555

EDIT: Also, the link preview images on older toots just stop working after a while, apparently. Neat.