feat: add Dropbox email module

[beranka]

Adds an email-existence scan module for Dropbox (its home page).

How it works

• Retrieves the CSRF token from cookies set when requesting the main page.
• Uses Dropbox's endpoint that explicitly returns whether an email is registered.
• The endpoint also returns whether the email is used as a recovery email for another Dropbox account and whether the associated account has a passkey configured.

Testing

Manually tested (and got expected results) with:

• An email that is not registered.
• An email that is only registered as a recovery email.
• An email that is registered and the associated account does not have a passkey.
• An email that is registered and the associated account has a passkey.

[json-hunter07]

@kaifcodec LGTM.
The last commit fixed the logic error you were talking about.

[kristoisberg]

Looks solid, LGTM too!

[kaifcodec]

@beranka Thank you for the PR, this one really is a solid addition as it reveals some extra data from emails. Feel free to add more new sites to the project.
MERGING IT!

@json-hunter07 @kristoisberg Thank you for the review on this.

[tiehardoim-coder]

FaEr