Allow users to search in encrypted room locally [#783].

[toshanmugaraj]

• Search is now available only for unencrypted rooms, and not available for encrypted rooms.
• Search can be enabled for encrypted rooms by searching from locally cached datas.
• For now the search can work within locally available data, and may be enhanced to efficiently load the entire history on users need.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request updates CHANGES.rst
• Pull request includes a sign off