[Accessibility] Remember hashtag casing from user input where possible

[marrus-sh]

Related to and complements #9912, but pertains more to the frontend than the backend.

In some cases, the casing of a hashtag is able to be determined from user input alone, because the user typed it exactly. This should take priority over all other possible casings, as it is a clear user preference. Some places where this can be implemented are:

• In the hashtag autocomplete (this was fixed in [Accessibility] Screen readers like #HashtagsLikeThis, but the Mastodon UI only offers #hashtagslikethis #8241)
• On public hashtag timelines (the case found in the URL should be used)
  • If this is implemented, then the casing of Mastodon‐generated links to public hashtag timelines (ie, in posts) should match the casing found in the posts where they appear.
• When doing a search, and receiving an exact match (the case used in the search should be used)
  • Additionally, this case should be carried over to the title of the hashtag timeline column when the search result is clicked.
• (more generally) When clicking on a hashtag in the web UI, the casing of the hashtag should be remembered as the title for the resulting column which opens (preserved via the /web/timelines/tag/… URL)

As an exception, if the hashtag provided by the user is entirely lowercase, the casing provided by #9912 should probably be used instead, to accommodate machine‐generated input (which often does not take casing into account). And, of course, the casing provided by #9912 should always be used in instances where casing cannot be determined by user input alone; for example:

• In the profile directory
• Inexact search matches
• Et cetera