Replace zoekt feature_flags with application_settings

What does this MR do and why?

Add Zoekt settings under the Advanced Search settings. Replace the three feature flags with the Zoekt Application settings.

Feature flag -> Zoekt application settings

• index_code_with_zoekt -> zoekt_indexing_enabled
• zoekt_pause_indexing -> zoekt_indexing_paused
• search_code_with_zoekt -> zoekt_search_enabled

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Make sure Zoekt is set up.
2. Run the gdk locally and navigate to /admin. You might need to restart the gdk. Notice that there is a new item Exact Code Search in the Settings
3. Open a rails console and verify that checkbox values match with the ApplicationSetting.last.zoekt_settings
4. Check/Uncheck the options and verify the changes reflected in the record ApplicationSetting.last.zoekt_settings

Related to #389765 (closed)

#452340 (closed)