Add overview dashboards for every Importer

Currently in order to get any statistics on Import/Export it's possible to do so, however you need to dig into sidekiq dashboard, find appropriate jobs, etc. Or analyze kibana logs and visualize them. It would be nice for each ~"group::import" importer to have a separate overview dashboard, that would answer the following:

• Current number of Imports
• Current number of Exports
• Avg (median?) Import/Export duration, 90th, 95th, 99th percentile, max duration
• Number of failed import/exports ?

There is Github Imported dashboard already available https://dashboards.gitlab.net/d/2zgM_rImz/github-importer?orgId=1 but it would be good to cover all importers:

• Bitbucket Server/Cloud
• Project Import/Export
• Group Import/Export
• Fogbugz
• Gitea
• By URL

Alternatively all of this can live in one dashboard. Might be easier to have an overview this way.