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

[jwodder]

(Requested by @yarikoptic in dandi/dandi-cli#1414 (comment))

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).