i18n: consistency tweaks

- Replace "can not" by "cannot"
- Add "Special:MyLanguage" to mediawiki.org URLs
- Add full-stop

Spotted by Siebrand
https://translatewiki.net/wiki/Thread:Support/About_MediaWiki:Uploadscriptednamespace/en

Change-Id: Ice4ab37fd2e680e9903643fd6f88e60a44787e2b
Merge "i18n: consistency tweaks"