Add support for computing repository storage statistics

This adds a new tool, storage_stats, to gather statistics on the used storage, as well as a new dashboard page to display the statistics. Due to exceedingly poor performance from attempting to use pure Python to parse all the packaging files in over 2000 snapshots (a total of several thousand files to parse & process), Rust was used for the storage_stats implementation; on my system, it determines the full extent of storage usage in under 20 minutes.

On the dashboard's side, the common HTML and styling was moved into a single shared template that both the current index/packages page and the new storage page can use, and navigational links were added to the pages.

https://phabricator.apertis.org/T8197

Signed-off-by: Ryan Gonzalez ryan.gonzalez@collabora.com