insights: [FE] implement gql API for insights creation UI (create, edit pages)

Created by: vovakulikov

Context

We have to implement a few API methods to support our current insight creation UI pages

• getInsight (actually two methods for code search insights and code stats insight pages)
• getDashboards method for visibility section (according to our latest approach on how we share insight)
• move form validation handlers to API context class (repositories validation, repositories suggestions search)
• createSearchInsight and createCodeStatsInsight mutations
• updateSearchInsight and updateCodeStatsInsight mutations