%link moniker for localisable links in ts calls

To handle non-English documentation pages we should implement %link moniker for ts() calls:

Instead of having

{ts 1=http:...}

Visit %1

{/ts} we can have {ts link=http:...}Visit %link{/ts}

(in the same manner we have %count now). The extractor should extract the string in the same way it does now, but should also create civicrm-links.pot file which holds the http:... links.

If Confluence can be taught about the relation between different language versions of a given page, then the civicrm-links.po files should be auto-generated from Confluence data.