[TENE-634] It takes 18s to display the list of sources

Added index for source entity (name, url). This is data, that sources grid uses during callbacks. Each call to db slows performance significantly. I've prepared local setup with fbc-test dump database that had the same issues, the paas pod one had. After adding index, performance increased, and now, using harvester-manager at fbc-test should be much more efficent. (similar user-experience like on fbc-dev) If performance still will be poor after addition of the index, we can tuning postgres params. This will also do the work because i've tested it before and performance increased directly on paas and on localhost.

However, i don't want to play with image settings because it could lead to some other problems in the future. I think index will do the job because on local enviroment it did. This is just plan B.