Add min char limitation to search autocomplete backend

Background

Search autocomplete endpoint does not enforce min character limit like /search endpoint

There are three types of autocomplete data returned when a user types into the search bar:

1. generic results
   • results for Settings and Help
   • these are hardcoded
2. recently viewed items
   • epics, issues, merge requests
   • these use Redis
3. resources
   • groups, projects, users, issues
   • these use a mix of Database and Elasticsearch

Proposal

Enforce the min character limit for resources autocomplete data since that hits the database and Elasticsearch. The search_autocomplete_opts method has three filter types (search, generic, and default) which return a variety of data.

• Combine the two character limits used in search params and search abuse detection
  • remove Gitlab::Search::AbuseDetection::MINIMUM_SEARCH_CHARS
  • replace use with Gitlab::Search::Params::MIN_TERM_LENGTH
  • change Gitlab::Search::Params::MIN_TERM_LENGTH to 2
• The resource_results method should return an empty array if the term length is less than the min (use Gitlab::Search::Params::MIN_TERM_LENGTH)