Fix minor IE UI issues

Closes #4088

- Fix Flexbox issue with the user list
- Fix positioning issue with the mobile menu toggle icon
- Make the mobile settings nav use absolute positioning (as fixed elements don't move inside a `transform: translate(...);` parent
Fix uncentered no posts view

Closes #4097
Merge pull request #4100 from novaugust/iss4097

Fix uncentered no posts view
Merge pull request #4090 from PaulAdamDavis/ie-fixes

IE UI fixes
More robust deprecation check code

Closes #4082
* reformatted code to allow for traversal
* deeper config items should be denoted like this: `object.object.object.property`.
* added tests for testing the deprecation warnings
Deprecated mail.fromaddress, mail.from is Title <email@address>

Closes #4018

* cleaned up `mail_spec.js`
* deprecated `mail.fromaddress`
* implemented 'Blog title <email@address.com>' format with fallbacks
* added tests to deprecation and from address, made existing ones more robust
* moved domain intuit into its own module: `GhostMailer.getDomain()`