Add missing translation keys and improve technical events description in text version of email notifications

Steps to reproduce:

1. Have an user with a valid email address set and following the entire wiki on Notification preferences
2. Login as Admin
3. Create/update/comment/delete a page
4. Mention the user in a page
5. Like a page
6. Trigger the "Notifications daily email" job from Scheduler.WebHome
7. On the user's received email, click on "..." (More) > '<> Show original' to see the text version of the notifications email
8. Observe the displayed events under the Event(s) on the wiki: section

Expected results

All events have user friendly and simple described events, like there are already:

• for Create: [create]
• for Update events: [update]
• for Delete: [delete]

Actual results

• for Comment there is: [addComment]
• for Mentions there is: [mentions.mention]
• for Like there is: [org.xwiki.like.events.LikeRecordableEvent]

which are a bit technical not quite user friendly.

Also, instead of Pages header for each event, only on Like and Mentions there is displayed:

• for Like: org.xwiki.platform:xwiki-platform-like-notifications
• for Mentions: org.xwiki.contrib:mentions-notifications

Please see the attached screenshot.

The issue is not new, reproducing also on previous XWiki versions.