Treat 304 as valid, add mock tests, fix mock path issue

[samford]

This continues the work started in #897.

Redirections

Regarding the discussion in that PR, 301, 302, 303, 307, and 308 redirections are handled by reqwest and the response.status().is_redirection() conditional block in check_url() wouldn't be reached under those circumstances. However, 300 (Multiple Choices), 304 (Not Modified), 305 (Use Proxy), and 306 (Switch Proxy, deprecated in HTTP 1.1) still reach that conditional block, since they're not handled by reqwest. As such, I've decided to keep the is_redirection() conditional block.

At least to me, the 304 response seems like it would be okay to treat as a valid response, so I modified is_valid() and check_url() to treat it like a success response. If you disagree, I can remove these changes and it will be treated like an error. Either way, it will likely never be encountered in practice, since it's sent in response to a request's If-Modified-Since or If-Match headers (which we're not using here).

Added Tests

I added link_checker tests for 301-to-200 status codes, 301-to-404 status codes, and 500 status codes, to test redirections and the previously-added response.status() checking for non-success status codes in check_url().

Mockito Path Collisions

While adding the redirection tests, I encountered an issue with Mockito, where mocks with the same path (e.g., mock("GET", "/test")) can potentially bleed between tests, so you may end up with an unexpected response which causes the test to sometimes pass and sometimes fail. To avoid this, I made the mock paths unique across tests, using pseudorandom alphanumeric paths. The current path naming scheme makes it easy to add new tests (without having to keep all the existing path names in your head) but if you would like to go about it a different way, just let me know.

The path names should be kept unique going forward as tests are added or changed, so I added a note above the tests in /components/link_checker/src/lib.rs and /components/templates/src/global_fns/load_data.rs. I may try to create an example to demonstrate the issue and see if the Mockito folks will look into it but this is the current state of affairs, at least.

Etc.

Besides this, I fixed Clippy warnings about using String::from() around format!(). I forgot that format!() returns a String (not a str), so the String::from() was unnecessary.

[Keats]

Thanks!