Make Constants.SEARCH_TIMEOUT_MILLISECONDS configurable

In the channel switcher, the webapp waits for Constants.SEARCH_TIMEOUT_MILLISECONDSto accumulate the letters and sending off the autocomplete request to the server. This value is currently hardcoded to 100ms with no way to configure it.

The problem is that unless a user types extremely rapidly, the first few letters will always spawn search requests. But then by the time the user finishes typing the search query, more requests go out, and come back faster with results because the match fewer results.

But the original requests are still being executed on the server, creating high latencies, and skewing the Grafana charts. This creates a lot of confusion for someone looking at the graphs. To fix this, we could increase the wait time to a higher value, or even better, make this a configurable setting.

The changes that need to be made for this are:

1. Add a config setting for DisplaySettings.SuggestionDebounceDelay to the Config type on both the server (model.Config) and the client (Config and AdminConfig). It should default to the current value of Constants.SEARCH_TIMEOUT_MILLISECONDS.
2. Expose the config setting to users by adding it to the data returned by GenerateClientConfig on the server.
3. Replace all usage of Constants.SEARCH_TIMEOUT_MILLISECONDS on the web app with the value from the client config.
4. Make the new setting configurable by adding it to admin_definitions.tsx in the web app. It should go under the Experimental Features section and have the following text:
   • Title: Suggestion Debounce Delay
   • Description: The number of milliseconds to wait after a user has stopped typing before showing autocomplete options or searching for suggestions. Affects components such as the message textbox, the Channel Members list and the Browse Channels modal.
5. Submit a pull request to https://github.com/mattermost/docs/ to add documentation for the new setting there as well.

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JIRA: https://mattermost.atlassian.net/browse/MM-60565