Add next page button

[qminlh]

Added a "Next page" button.

Technical details

First of all, in the description below I'm mentioning "search data" - it's a JSON object that can be defined as follows (TypeScript-style definition):

{
    options: {
        query: string,  // search query
        bookmarks?: string[],  // undefined for first page; if defined, it's an array containing a single string that is a reference to current position
        // irrelevant stuff
    },
    // irrelevant stuff
}

Requests for new images are made to the URL "https://www.pinterest.com/resource/BaseSearchResource/get/". First request is done as a GET method and contains URL-encoded search data as a query parameter with key data (that part is already implemented), while further requests are done with POST method and the query parameter is moved to request body together with the key, with encoding unchanged.

In POST requests, search data also needs to contain bookmarks - an array containing a single string, which serves as a reference to user's current position/page. The string is located in previous response, in .resource_response.bookmark. These requests also need a csrftoken cookie, which is received on a first request.

Implementation notes

• I store CSRF token in the URL and not as a cookie, so that by design a new token is generated with each search - this prevents fingerprinting.
• Response object contains the entire bookmarks array in .resource.bookmarks - it's mostly the same as the string that I used, with a different behavior only on last page (.resource_response.bookmarks becomes undefined, while .resource.bookmarks changes value to ["-end-"]). Choosing .resource_response.bookmark was an arbitrary decision.

Additional notes

• I also added "www." to the beginning of the URL to avoid unnecessary extra request, as the pinterest.com URL redirects to www.pinterest.com.
• Full-fledged pagination may or may not be possible - research on the structure of bookmark is needed.
• I don't have much PHP experience, so I would strongly recommend a security-oriented check.

Fixes #2

[Ahwxorg]

Is the XSS vuln patched in the latest commit? @qminlh

[qminlh]

Is the XSS vuln patched in the latest commit? @qminlh

Yes

[Ahwxorg]

Alright, merging it :)