Remove html from "Posted on" string

This reduces the amount of HTML in the string. It should be no problem splitting the two strings as they both can be used on their own and still make sense.

@translators: My suggestion here is to change the string from:
`<span class="posted-on">Posted on %1$s</span><span class="byline"> by %2$s</span>`
to:
`Posted on %1$s` and `by %2$s`

Please let me know in the comments if it would work or not in your language.