Revert "remove direct links to external sites from source templates, fixes #195"

This reverts commit 44a04a22a32cd5df84476dd6afdc7aa74464f730.

Conflicts:

	securedrop/source_templates/howto-disable-js.html
More external links
Add custom redirector to clear referers

We removed external links in #206 because Tor Hidden Services leak
referers (https://trac.torproject.org/projects/tor/ticket/9623).
Obviously this isn't ideal because of the usability tradeoff.

One way to get around this without using Javascript is to replace all
external links with a link to a blank SecureDrop page that does a meta
refresh to the external link, since meta refreshes always clear referer.
Use custom redirector for external links
Avoid namespace pollution, add comments
Merge pull request #246 from diracdeltas/feature/external-links

Use meta-refresh to clear referer, add external links back