stats: wait until blockscout finishes indexing

[sevenzing]

For now we have to manually wait until blockscout finishes indexing, and only after that start stats service (overwise statistic will be incomplete). This is super inconvenient, so
I suggest:

1. In stats determine if blockscout is currently in indexing status and just wait + return {"status": "WAITING_FOR_BLOCKSCOUT_INDEXING"} instead of {"status": "SERVING"} in /health
2. In frontend: show Stats and charts button in the menu, but in stats page go to /health endpoint and in case WAITING_FOR_BLOCKSCOUT_INDEXING show page like "We are currently indexing blockchain data. Please wait until blockscout finishes indexing."

[bragov4ik]

The waiting part is working; reporting extracted to a separate issue #1123