Stats on a per-tag basis

[cesswairimu]

Implement display of stats on a per-tag basis, via standard model functions with unit tests

• Move tag graphs to an individual page(this page) Hide tag graph from tag pages like /tag/ui with a toggle button #4646
• Display stats for each tag

[jywarren]

Noting that the tag graph we're looking to move off of individual tag pages (#4593) could be the very first feature of this page!

[jywarren]

[jywarren]

Also noting from #4593 that we should be sure to have the graph axes and scales labeled as this page comes together. I think @cesswairimu has been looking at a new graph library that may make this easier!

[jywarren]

And the graph template is here: https://github.com/publiclab/plots2/blob/master/app/views/tag/_graph.html.erb