escape gravatar URLs to comply with W3C standards

This fixes an HTML validation error due to & not being properly escaped on the gravatar URLs.
fixes user specs about ampersands in gravatar URLs
add pluralization rules in ember for Korean (ko)
BUGFIX: caching sass is not valid due to "import" being used
BUGFIX: backups logs page was busted since last Ember update
Update README.md
Update README.md
FEATURE: warn the admin whenever we disable the download_remote_images_to_local site setting
display the copy & paste images hint to FF users too
Merge pull request #2288 from yomybaby/i18n_plural_ko

add pluralization rules in ember for Korean (ko)
slightly improve disk space limit reached message
FIX: Admins should always be able to see groups so they can edit them.
REFACTOR: Add urls for admin groups, make it more idiomatic ember
FIX: NotifyMailingListSubscribers can handle trashed posts, instead of raising an error and retrying
Add a section about Translations in CONTRIBUTING.md
Update .tx/config for the new transifex project url
FIX: Markdown formatting error
Add comments to the top of client.en.yml and server.en.yml about Transifex stuff
Allow contact user to send private messages even if enable_private_messages is false
BUGFIX: category name is reserved in multisite