Integrate with external services

[yarikoptic]

Inspired by

• Show links to version & asset metadata in web view #13

If to add all kinds of hooks into dandi archive etc, we better also then expose integration with external services like we already do for datasets.datalad.org and dandi-archive. To the moment we do not have unified registry (as originally wished for in https://github.com/con/external-services) but we have a growing number in dandi-archive at https://github.com/dandi/dandi-archive/blob/HEAD/web/src/views/FileBrowserView/FileBrowser.vue#L319 -- would just need to be extracted from Vue into json for reuse.