Add X-Url-Delete-* headers to POST handler

[anihm136]

Fixes #419
Adds X-Url-Delete headers to the post handler. Since the post handler currently supports multiple file uploads (as multipart file form), currently the header is postfixed with the token of the file to create a unique delete token for each file (any better suggestions for handling this can be implemented)

[aspacca]

any better suggestions for handling this can be implemented

We could send multiple X-Url-Delete headers, with every delete url.
Replacing
w.Header().Set("X-Url-Delete-"+token, resolveURL(r, deleteURL, s.proxyPort))
with
w.Header().Add("X-Url-Delete", resolveURL(r, deleteURL, s.proxyPort))
should be enough

The delete url already contains the full get url including the filename https://transfer.sh/QMXz5o/hello.txt/e199GfHtnsyC so users should be able to match

[aspacca]

@anihm136 are you still going to work on this?

otherwise I will create a new branch with the fix, thanks :)

[anihm136]

Sorry, I'm a little packed right now. Please go ahead with the fix

[anihm136]

Hey @aspacca I have some time on my hands for the next week or so. Shall I go ahead and finish this?

[aspacca]

@anihm136 thanks

could you fix the conflict with main branch?

there was some refactoring since you opened your branch

[anihm136]

Should be done

[aspacca]

@anihm136 can you fix the linting?

thanks

[anihm136]

Done. Unfortunately I missed it because my editor uses the --fast flag with the linter, which missed the error

[stefanbenten]

We should not need this call here, due to the defer in L319.