[Part 6] Add a DebouncedCache utility for frontend

[MareStare]

Before you begin

• I understand my contributions may be rejected for any reason
• I understand my contributions are for the benefit of Derpibooru and/or the Philomena software
• I understand my contributions are licensed under the GNU AGPLv3

• I understand all of the above

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Debouncing and Caching

Today, there is an annoying implementation behaviour of debouncing and caching for server-side completions. We debounce user input with a 300ms threshold, then send a server-side completions request and cache its result. However, if the user changes the input and then returns back to the cached input, the user still needs to wait for a 300ms debounce threshold before the cached results are displayed.

I fixed this behaviour and also moved this logic into a generic DebouncedCache class. It also manages an AbortController/AbortSignal, so that it can cancel the processing after an HTTP request was invoked if the user typed some more characters in the middle of that. So this change eliminates another race condition, where previously the user could receive stale sever-side completion results while they were typing. It isn't a critical bug though, but still nice to see it fixed.