[TopicSpace] Show list of autocompleted topics that user can select

[lyqht]

Description

Currently in TopicSpace, users can choose either from a default list of topics or give their own topic input.

It will be good that as they type in the topic input, they are given a list of autocompleted topics just like the behavior of adding topics on GitHub manually to each repo.

Refer to screenshot below for the API call is made and how the UI is like for the autocomplete on GitHub.

Note that the API call is only made after user stops typing for a short while in the user input.

Suggested solution

• Add API route that calls GitHub's autocomplete API based on user's input
• UI to use the grouped option picker, with the group name being "Suggested"
• UI to update the options available to be based on user input + GitHub's autocomplete API result.