Migrate Grafana from using SQLite to PostgreSQL

User story:

• As PMM developers, we want less complexity in the system with many different DB's used inside
• as PMM is used, we need the better performance of Auth system and sessions. It's a known problem with Go and SQL like about concurrency, so the SQL lite is a problem for deployments with a significant number of users and services

UI/UX:
N/A

Acceptance criteria

• PMM Grafana storage changed from internal SQL lite to PG used inside PMM for Inventory data
• All data migrated correctly and change of the component is transparent for the user and does not require additional actions from them, besides usual update

Out of scope:

• TBD

Suggested implementation:
TBD

How to test:
It's quite a big change. It contains two parts:

1. Change database encoding to UTF8
2. Migrate to Postgres database for Grafana. 

All PMM functions can be impacted. What I think is worth paying attention to when testing:

1. Upgrade process. Especially with data (Mongo, Postgress, RDS instances).
2. Adding/Editing new instances.
3. Grafana works as expected (Adding new users, editing/creating dashboard, datasources, and so on).

Original report:

With a lot of authentication requests, sqlite3 locks too often. We need to use PostgreSQL instead.

For PG we need to use separate DB ('grafana')

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This will give us much better stability, especially for User Auth requests, and probably eliminate bugs with logged out users