Notification center: review the messages strings

[afercia]

A few messages in the notification center are actually built using two strings separated with a line break. Not sure if there's a specific reason for this, but probably it's something that should be avoided to give better context to translators and also for layout purposes, especially if the notification center is going to be fully responsive.

[afercia]

@tacoverdo when you have a chance: should these strings be unified in a single string in your opinion?

[afercia]

This still happens on current trunk. Some notification text is made of 2 strings separated by a <br />:

[Djennez]

This is currently still in Yoast 9.7 in the following functions:

• page_comments_notice()
• permalink_notice()

[afercia]

Looking back at old issues, I'd like to add a quick screenshot to better illustrate why forcing line breaks is not ideal for responsive design:

[afercia]

Though minor, this is still a problem on current trunk.

Line breaks in a long text may look OK on large screens. But on small screens things are pretty different and they just look not OK: