Plugin install endpoint: slug should never be empty

the plugin slug could have been empty if the plugin was a single file,
and not stored in a directory
Plugin Install Endpoint: use slug in the request

Instead of using the plugin id ( i.e. wordpress-seo/wp-seo ) in the request
we will use the slug ( i.e. wordpress-seo ). In browsing plugins on the .org
directory, we found that only the slug is available. I created a new function
get_plugin_id_by_slug, so that, after installation, we can convert the slug
into a plugin id, so that our newly installed plugin will pass validation
REST API: Site updates status. Return sites jetpack version.
Tiled Galleries: fix typo in gallery attributes
Readme: change contribute link from Trac to GitHub
Pull down translations from Glotpress.
Merge pull request #1218 from Automattic/fix-issue-1217

Plugin install endpoint: slug should never be empty
Pull translations.
Don't enqueue scripts before init. Ever!
REST API: Plugin Delete Endpoint:  do not delete an active plugin.

The WordPress dashboard does not allow one to delete an active plugin,
neither then should the API.

+ the plugin slug could have been empty if the plugin was a single file,
and not stored in a directory

+ Instead of using the plugin id ( i.e. wordpress-seo/wp-seo ) in the request
we will use the slug ( i.e. wordpress-seo ). In browsing plugins on the .org
directory, we found that only the slug is available. I created a new function
get_plugin_id_by_slug, so that, after installation, we can convert the slug
into a plugin id, so that our newly installed plugin will pass validation

+ Removing some unneeded properties

props @roccotripaldi
Updates to the way we present Jetpack features.

- simpler tag line
- new section including feature groups
- shortening text of the long feature list bullet points
- removal of various distracting links
- fix various typos
- link to installation video walkthrough
Revert "Updates to the way we present Jetpack features."

This reverts commit 26ff063a32401e225bb444ad748223d94418bc5d.
Updates to the way we present Jetpack features.

- simpler tag line
- new section including feature groups
- shortening text of the long feature list bullet points
- removal of various distracting links
- fix various typos
- link to installation video walkthrough
Tweak to tag line.
Adds Oxford comma!
Language updates.
REST API: Don't check full management on GET requests

Informational things should come out.
Merge pull request #1228 from Automattic/fix/dont-check-fullmanagment-on-get

REST API: Don't check full management on GET requests
Merge pull request #1229 from Automattic/updated-readme

Updated readme
Version bump!