Fix live theme switching not working on subdirectories

Closes #1770
- Previously, the middleware would check that the route on the stack was an empty string, which worked when there was no subdirectories
- When subdirectories were added, the proper route was only set when
  updating the theme
- Because it was only set when updating, this explains themes working on
  initial load, since the stack location was looking for an empty
  string, which is what the middleware was initialized with
- However, once a new theme was set, it was still look for an empty
  string, which would never exist, which caused the issue
- Now, the route is properly set on initialization of the middleware,
  and then the `config.paths().subdir` property is used for the check
Update import tool to be safe

closes #1681

- import doesn't override user credentials
- import doesn't override theme
- import doesn't kill session
- import does refresh the settings cache
- updated tests, they now use a fixture instead of a generated export
- tests check to ensure import is safe
Merge pull request #1780 from ErisDS/issue-1681

Update import tool to be safe
Merge pull request #1784 from remixz/theme-switching-subdirs

Fix live theme switching not working on subdirectories
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
Fixing notifications on ugly debug page

- also fixes an issue where the debug tools didn't redirect properly when Ghost was operating in a subdirectory.
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