Fix tag links for terms with special characters (e.g. C#)

[markheydon]

Summary

• Single-page tag links were built with urlize on the raw tag string, which strips # (and mishandles similar characters), so C# linked to /tags/c and 404'd.
• Switch the tags sidebar in layouts/_default/single.html to .GetTerms "tags" + .RelPermalink + .LinkTitle so links match Hugo’s generated taxonomy term pages (e.g. /tags/c%23/).
• Preserves existing markup/classes and keeps openInNewTab default as true.

Fixes #261

Test plan

• Add a post/project with tags: ["C#"]
• Build/serve the site
• On the single page, click the C# tag link and confirm it goes to /tags/c%23/ (not /tags/c)
• Confirm other ordinary tags still link correctly
• Confirm tags still open in a new tab by default

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