[keycloak-dev] First round of unifying application and oauth clients

First round of unifying application and oauth clients is done. What I've done is: * Combined ApplicationModel and OAuthClientModel into ClientModel * Removed OAuth Clients (html and js) from Admin Console and Admin Endpoints * Renamed Applications menu entry to Clients in Admin Console Next steps are: * Add database migration * Update Representations (deprecate OAuthClientRepresentation and ApplicationRepresentation and add ClientRepresentation) * Rename Application to Client in Admin Endpoints and Admin Console html/js * Look for other references to Application or OAuth Client in code * Update documentation, including migration guide