add stylelint and fix all less files

.stylelintignore

.stylelintrc

.travis.yml

package.json

...cketchat-action-links/client/stylesheets/actionLinks.less

...ketchat-authorization/client/stylesheets/permissions.less

...tings-mail-messages/client/stylesheets/mail-messages.less

...hat-emoji-custom/assets/stylesheets/emojiCustomAdmin.less

packages/rocketchat-emoji/emojiPicker.less

...ketchat-integrations/client/stylesheets/integrations.less

...ocketchat-livechat/app/client/stylesheets/_variables.less

...s/rocketchat-livechat/app/client/stylesheets/loading.less

...ages/rocketchat-livechat/app/client/stylesheets/main.less

...echat/app/client/stylesheets/utils/_keyframes.import.less

...ages/rocketchat-livechat/client/stylesheets/livechat.less

...ge-attachments/client/stylesheets/messageAttachments.less

...chat-message-pin/client/views/stylesheets/messagepin.less

...-message-snippet/client/page/stylesheets/snippetPage.less

...at-message-star/client/views/stylesheets/messagestar.less

...oauth2-server-config/oauth/client/stylesheets/oauth2.less