[BUG] Missing attributes for external links created in Markdown tiddlers

[gera2ld]

Describe the bug External links created in Markdown tiddlers do not have some attributes as in normal tiddlers. As a result, the links are not properly styled when appearance is changed.

To Reproduce Steps to reproduce the behavior:

1. Go to https://tiddlywiki.com/plugins/tiddlywiki/markdown/#HelloThere
2. Optionally inspect an external link, say Remarkable, we can find <a href="https://github.com/jonschlinkert/remarkable" class="tc-tiddlylink-external" target="_blank" rel="noopener noreferrer">Remarkable</a>.
3. Click on MarkdownExample to open a Markdown tiddler.
4. Optionally inspect an external link there, say syntax page, we can find <a href="https://daringfireball.net/projects/markdown/syntax" target="_blank">syntax page</a>.
5. Now change appearance, open $:/ControlPanel, Appearance > Palette, pick Gruvbox Dark.
6. Look back to the links in the Markdown tiddler, they are hardly seen.

Expected behavior Links created in Markdown tiddlers should have the same attributes as in normal tiddlers. The class attribute is essential since it affects appearances.

Screenshots

Desktop (please complete the following information):

• OS: macOS Catalina 10.15.6
• Browser: Chrome
• Version: 85.0.4183.102

[Jermolene]

Thanks @gera2ld