fix: disallow updating files for links and links for files

[halfwhole]

Problem

One can manually submit a request (PATCH /api/user/url) to update the longUrl for a file, or the file for a link. This should not be allowed.

Solution

Disallow updating files for links and links for files at the service layer, depending on the isFile field of the URL.

• Return a 400 bad request on the user and API controller when encountering this error
• Created a new error type InvalidUrlUpdateError for this, because it didn't seem to fall under any existing error type - not sure of my approach though, happy to modify where necessary

[thanhdatle]

LGTM