Bugid 70020, 70081, 64316 - help page link tweaks for video tools

The attached commits address three tickets:

https://wikia.fogbugz.com/default.asp?70020 and https://wikia.fogbugz.com/default.asp?70081 - Both the VET and the AddVideo tools had  more or less hardcoded links to  help pages listing supported sites from which a user can embed videos. I created new messages which contain the help page URL and replaced the hardcoded links with references to proper messages. This solution is more i18n-friendly because it allows the i18n team to customize where the links go to, which also resolves https://wikia.fogbugz.com/default.asp?64316. If they are not translated, they will still default to English.

I'm not sure about the syntax and if the commits are compliant with any coding practices Wikia is using, that's why I'd like to ask an engineer to review the code and if everything is ok - merge with the main branch. Thanks in advance! :)