[#566] add optional 'search' query param

[jankun4]

Allow for searching for specific GA by title, motivation, about and rationale

List of changes

• Add
  optional search query param to proposal/list

Checklist

• related issue
• My changes generate no new warnings
• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the changelog
• I have added tests that prove my fix is effective or that my feature works