Jetpack: Holiday Snow: Remove the snowflake from the adminbar.

In hindsight, looks like it was more confusing than helpful.

Revert "Jetpack: Holiday Snow: Add an adminbar item of a snowflake if the user can control the snow being active, they are in wp-admin, and it is currently "

This reverts commit 30fca55f265f7fe0e08ea9265f150be0aac0ef11.
Adding in blank files for backcompat with anything manually including them.

Fixes some errors that have cropped up like:

```
Warning:
require_once(/home/private/public_html/wp-content/plugins/jetpack/module
s/featured-content/featured-content.php) [function.require-once]:
failed to open stream: No such file or directory in
/home/private/public_html/wp-content/plugins/jetpack/modules/theme-tools
.php on line 54
```

Introduced in
https://github.com/Automattic/Jetpack/commit/7a3e6bf205e08c2ffa4a2b4514f
6513a875c4d51
Add compat file support for twentyfourteen

Add into theme-tools.php the ability to load a theme-compat file, and
make it extensible so that themes or plugins can add or remove them.
Use class_exists() guard for Featured_Content.
Merge pull request #1 from tierra/feature-content-guard

Use class_exists() guard for Featured_Content.

Necessary for wp-cli compatability, as wp-cli loads plugins -after- themes, as opposed to before like core does.

Implementing this in theme-tools.php rather than theme-tools/featured-content.php so as to maintain the file integrity of featured-content.php against its wpcom variant.

Props @tierra
Merge pull request #4 from Automattic/2.7.1

Merge the changes in 2.7.1 upstream to master.