v3: route namespacing

• other than /api, /api/v3 can be used as a prefix (API docs will highlight the preference for it)
• removed /api/limits, superseeded by /api/profile
• /api/{login,apikey,revoke,register,recover_username} => /api/auth
• /api/d1check => /api/d1/check
• DELETE /api/account => DELETE /api/profile (suggestions welcome)
• admin.{domains,settings,misc} now use the prefix (no changes done api-wise)
• finish admin blueprints

closes #120 (closed).

keep in mind /api/upload and /api/shorten will stay as is to cause as less of a disruption as possible.