Swagger should be reserved when creating users (or: move /swagger to a different path)

[thehowl]

• Gitea version (or commit ref): master
• Can you reproduce the bug at https://try.gitea.io:
  • Yes: https://try.gitea.io/swagger/this-exists - https://try.gitea.io/swagger

Description

At the moment, if you try to access /swagger, it will not show the organization I created, but the org does exist. In order to address this issue, we should either forbid users creating new orgs or users with the name swagger (adding it to the list of blacklisted usernames), or move /swagger somewhere else (but thus generating some linkrot on gitea instances - perhaps redirect to new URL if there is no org, and show the org if there is one)

[strk]

I'd prefer moving swagger to /api/swagger or even /api/<version>/swagger when it makes sense