[bugfix] Allow blocked accounts to show in precise search

[tsmethurst]

Description

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This pull request updates the search implementation to allow showing accounts in results that have been blocked by the searcher, but ONLY if the search query is specific enough that it's unmistakeably the blocked account that's being searched for.

Previously we were a bit too strict in our visibility checking, so if you blocked someone and wanted to unblock them, you couldn't just search for them and click unblock, you had to either go through your notifications, or mentions, or whatever, looking for the person so you could open their profile and click unblock on them.

Blocked accounts will still not be shown in more general search results.

Also does a little quality-of-life stuff:

• show generic avatars for blocked accounts, so that looking through all the accounts one has blocked doesn't just show empty avatars.
• return empty status slice rather than 404 when trying to view statuses of a blocked account (avoids errors in clients like pinafore)

closes #2255

Checklist

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[mirabilos]

Sorry this is too late, but would it be a good idea to take one of the generic avatars, overlay a 🚫 on top of it, saving that under a separate name, and using that? Since you already touched the avatar logic for this.

No worries if you’d rather not, just a quick idea.