FORUM-682 Incorrect labels depending on the language

Problem analysis:
* The notification box of "private message" and "private post" is quite similar in terms of parameters.
  However, in some language such as French, the words "post" and "message" have different genders and the adjective "private" must be agreed properly.
  The usage a common i18n parameter in the notification script isn't correct in this case.
* Use a common resource bundle key of last update information. This doesn't satisfy the case of "just now" update (less than 1s) in most of languages.

Fix description:
* Separate the i18n labels of private post and private message in notification parameters.
  Update ForumSendNotification.js and UIForumPortlet.java to initialize these notification parameters.
* Update UITopicDetail and UIViewTopic forms to handle correctly the i18n label of last update information.
FORUM-682: Separate i18n keys for the links to private post and private message
FORUM-888: [Forum] Escaping in Send Notification's i18n labels

Problem analysis:
* In the input message of Send Notification scripts there are i18n labels.
  These labels in some languages might need to be escaped following JavaScript String rules.

Fix description:
* Implement getLabelEscapedJavaScript method to escape characters following JavaScript String rules
* Use this new method before initializing Send Notification in JavaScript.
Merge conflict stable/4.0.x