Links in job posts should be validated

[jace]

Users sometimes forget to include the http:// in a link. The editor widget should validate all links in the post to confirm they are correct.

1. Force all URLs to be absolute (unless they are links to other posts on the job board, but how do we tell?)
2. Check if the link is valid (ie, it loads with status 200)

[jace]

Additional feature that can be implemented as an extension of this code: scan for banned link destinations, such as affiliate links to freelancer.com.

[jace]

Implemented via #77 in 345c307 and subsequent commits.