Fix and improve page number validation for following/followers pages

[pidgezero-one]

Closes #9283 (cont'd)

• Fixes page validation for followers/following pages in the event that the page ID is already an integer and not a string
• Defaults to the last valid page if the provided page is too high

Technical

• I extracted page validation into an "unexported" function for cleanliness. It covers the page qparam being either an int or a string as well as a fallback to just return 1.
• I noticed I had no checks for invalid pages that are too highly numbered, so I added that as well.

Testing

I repeated my tests from #9323, but also tried loading the page with invalid high values like 21, 100, etc.

Screenshot

There was no change in the results of my tests from #9323.

Here is another screenshot with a page number that is too high:

Stakeholders

@mekarpeles

Attribution Disclaimer: By proposing this pull request, I affirm to have made a best-effort and exercised my discretion to make sure relevant sections of this code which substantially leverage code suggestions, code generation, or code snippets from sources (e.g. Stack Overflow, GitHub) have been annotated with basic attribution so reviewers & contributors may have confidence and access to the correct context to evaluate and use this code.