Add customization options for enrollment and invitation e-mails

packages/rocketchat-lib/i18n/en.i18n.json

packages/rocketchat-lib/lib/placeholders.js

packages/rocketchat-lib/package.js

...s/rocketchat-lib/server/methods/insertOrUpdateUser.coffee

.../rocketchat-lib/server/methods/sendInvitationEmail.coffee

packages/rocketchat-lib/server/startup/settings.coffee

packages/rocketchat-ui-admin/admin/admin.coffee

packages/rocketchat-ui-admin/admin/admin.html

server/lib/accounts.coffee