Add welcome email for new sign ups

Closes #1766
- Once signed up, a quick welcome email is sent
- Links to their blog url, and gives the email they used to sign up
RSS uses correct links when dated permalinks are enabled

closes #1777
- Uses coreHelpers.url to resolve post url instead of assuming it is siteUrl + post.slug
- Functional feed tests now check for content instead of just rss tags
Merge pull request #1779 from remixz/welcome-email

Add welcome email (GH-1766)
Merge pull request #1785 from markberger/1777-rss-with-dated-permalinks

RSS uses correct links when dated permalinks are enabled
Don't allow a subdirectory called Ghost

fixes #1755

- adds extra validation to config loader
- adds tests for config loader and validation
Merge pull request #1781 from ErisDS/issue-1755

Don't allow /ghost/ subdir
Update connect-slashes to version 1.2.0