fix: don't apply recaptcha requirements when it's not configured

[davwheat]

Fixes #47

Changes proposed in this pull request:

This prevents ReCaptcha code from attempting to load, and prevents backend validation, when ReCaptcha isn't configured.

(God I'm rusty at this... let me know what I've inevitably missed!)

Reviewers should focus on:

Screenshot

With config:

Without config:

Confirmed

• [x] Frontend changes: tested on a local Flarum installation.
• [x] Backend changes: tests are green (run composer test).

[dsevillamartin]

(God I'm rusty at this... let me know what I've inevitably missed!)

Mood lol.

We might want to strip the settings of whitespace as well. Since the fields are required, users might set them to to bypass that.

[davwheat]

True, though I have no idea why someone would purposely bypass this...