Before, Gitea shows the database table stats on the `admin dashboard`
page.
It has some problems:
* `count(*)` is quite heavy. If tables have many records, this blocks
loading the admin page blocks for a long time
* Some users had even reported issues that they can't visit their admin
page because this page causes blocking or `50x error (reverse proxy
timeout)`
* The `actions` stat is not useful. The table is simply too large. Does
it really matter if it contains 1,000,000 rows or 9,999,999 rows?
* The translation `admin.dashboard.statistic_info` is difficult to
maintain.
So, this PR uses a separate page to show the stats and removes the
`actions` stat.
## ⚠️ BREAKING
The `actions` Prometheus metrics collector has been removed for the
reasons mentioned beforehand.
Please do not rely on its output anymore.
