Error handling improvements & test captcha in admin

[dsevillamartin]

Fixes #41 Fixes #30 Fixes #29

Changes proposed in this pull request:

• Use Guzzle for reCAPTCHA validation
• Pass errors during reCAPTCHA request to user (these errors don't reveal anything harmful, just codes for problems during request) so it doesn't just say validation failed with no other feedback
• Add some helper text to explain a bit more about the captcha type
• Add reCAPTCHA test section that uses current values in input fields
• Fix reply post validation not properly checking for whether the post is the first in a discussion

Screenshot

Confirmed

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

Required changes:

• [x] Related Flarum core extension PR's: (Omit this section if irrelevant)