Feature request: Allow users to assign posts custom hashtags

This is a new feature request to give users much more power in the ability to control and organize posts they see. In projects like Gitlab, I can add tags to issues that someone files so they can be easier to sort and organize. We should add a similar feature in smilodon (and send the improvements upstream) that allow users to apply custom hashtags to posts for their own use. When they see those posts, the posts will behave as though the poster had added the hashtags themselves. This would require changes to both the client and server.

A few rules for this:

• The poster cannot see the hashtags (otherwise people could use hashtags as a kind of graffiti on someone's post).
• Users can decide whether they want to see the custom hashtags of people they follow (like they can choose whether they see boosts).
• These are standard #hashtag hashtags, so they would work like any normal hashtag and can be searched for and filtered.
• From an implementation standpoint, the client-side should show a hashtag logo the user could click on that would display a pre-populated list of common hashtags, ultimately (v2) populated with hashtags they commonly use. They can select multiple hashtags to apply to the post.
• We would reuse whatever methods are already used to track boosts, to store a new type of record for hashtags.

Use cases:

Alice searches using the #devops hashtag to follow posts that are about that topic. She notices sometimes some people she follows forget to add that hashtag to posts on the topic, so she can add a custom #devops hashtag to those posts for her own use. People who follow Alice can choose to take advantage of her tagging and have the same ease-of-sorting.

Bob uses social at work and does not want to see sensitive content in his feed. He has added #nsfw to his filter list, but the #nsfw tag doesn't always get evenly applied and he would like to be able to tag content as sensitive if the poster forgets, without having to report posts to moderators all the time. Bob is a developer and with this new tagging feature, he was able to write a bot that scans through posts for nudity and tags it with #nsfw. He (and anyone else who wants the feature) can follow that bot and have a better chance of filtering out sensitive content.

Candice normally likes political discourse, but this election cycle is going on and on and she would just like a break for a week. People in her feed, including her, are pretty good about assigning the #politics tag to their posts, either directly or through the new hashtag feature. She adds #politics to her filter list for a week.

Donald loves that HBO series that everyone is talking about, but he's going to be out of town for the season finale. His community is good about adding #showname to posts about the show, and #spoiler to posts that contain spoilers. He adds #spoiler to his list of hashtags that don't get filtered outright, but like with #nsfw will treat the post as containing sensitive content, so he can read them later after he gets time to watch the season finale.

Eunice is concerned about the proliferation of hate speech on social media and wants to protect herself and her community from hate speech. While they only opt-into posts, sometimes they come across hate speech in comments or boosts or are made aware of them by people in the community. She and her community of followers adds #hatespeech hashtags to those posts and people who follow her or other prominent people in the group can now filter on #hatespeech and be protected from objectionable content.

I can come up with an example for each letter of the alphabet, but I think this shows some common use cases. The ability for users to tag posts for their own use, and have people in their network optionally benefit from that tagging, would give users a lot of extra power to control and organize their feeds both to find and organize topics they want to see, and filter topics they don't.