add Person clicking "Publish" to "publishedBy"

[yarikoptic]

As came up during dandi-standup.
ATM "publishedBy" is populated with an Activity "DANDI publish" which wasAssociatedWith "DANDI API" process.
Might require more discussion and how it should relate to overall "provenance tracking" of changes etc, but insofar I think publishedBy should ideally provide "Actor" (Person) information

[yarikoptic]

attn @waxlamp - might be a good issue to tackle while also working on #189