Replace `user=<STRING>` query parameters for `/dandisets/` endpoint with `mine=<true|false>`

(Requested by @yarikoptic in https://github.com/dandi/dandi-cli/pull/1414#discussion_r1507731964)

The `/dandisets/` endpoint currently takes an optional `user` query parameter that, when present, can only be set to `me` (in order to retrieve only the authenticated user's Dandisets). However, having a query parameter that can only be set to a single value doesn't make much sense. It would be better for the `user` parameter to be replaced with a parameter `mine` (or similar) that can be set to either `true` or `false` (default `false`).

Find the /dandisets/ endpoint implementation and its request tests; verify the current user=me behavior first. Update the endpoint contract so mine accepts true or false and defaults to false, then run the endpoint tests and confirm filtering the authenticated user's Dandisets works.

Written by the indexing model from the issue text.