Remove full stop/period from you_have_been_registered string in selected textpacks

Lessens the awkwardness of this incorrectly-formatted sentence in the welcome email:

```
You have been registered as a contributor to the site. My Site Name
```

Some textpacks had a trailing . after the `you_have_been_registered` string, others didn’t - this unifies them, to the best of my knowledge. It still doesn’t have a trailing . to indicate the end of the sentence proper (after site name), but it’s a start. The string `you_have_been_registered` string is currently only used in the welcome email.