Autocomplete: Extracting & slightly refactoring parts of the code

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
[x] I understand all of the above

This PR mostly attempts to split the code to reduce cognitive complexity.

All the logic related to displaying & interacting with autocomplete popup was extracted into SuggestionsPopup class.
- Now only one actual popup element is getting rendered and re-used for multiple inputs, instead of constantly removing and rendering a new ones every time.
Function of requesting the server-side suggestions was extracted into fetchSuggestions function.
- Caching now happens insight the function, only one request will be sent per term.
- Fixed the term not getting "normalized" before sending it to server-side auto-completion endpoint. Now the term is getting lower-cased and all white-spaces are trimmed out.
- Replaced the Object used for caching with Map. This was done to fix errors when attempting to search terms matching with default Object methods, such as: toString, constructor, __defineGetter__, __lookupGetter__, etc.

philomena-dev / philomena