Add loki logs dashboard to MySQL mixin (optional)

[v-zhuravlev]

This PR adds optional dashboard with loki logs to MySQL mixin. (off by default)

There are also dashboard links to quickly navigate between two main and logs dashboards.

• mixin: Conditional dashboard "MySQL Logs" that can be enabled/disabled by enableLokiLogs flag in config
• mixin: links to navigate between MySQL Logs and MySQL Overview with context
• mixin: renamed $datasource to $prometheus_datasource, added $loki_datasource
• mixin: Updated README with configuration snippets and sample screenshots

To enable logs support in MySQLd mixin, enable them in config.libsonnet first:

{
  _config+:: {
    enableLokiLogs: true,
  },
}

then run

$ make build