search contexts: improve creating and editing large contexts

Created by: novoselrok

Currently, it is only possible to create a search context by using repository IDs. On the client we required that the user enters repositories by name. This causes an issue with large contexts because we have to first fetch all repositories by name to get their ids, and then submit GQL request.

To improve on the client UX, I made the repositoryID field optional and added the repositoryName field (also optional). Now the client can make the request with either a repository ID or a repository name (mutually exclusive).