Cards improvements

[nikku]

Adds various minor improvements to the task board:

• Do not blindly stack all PRs, instead let PRs be part of an epic (child task), too
• Be able to link to an issue comment from within a tasklist
• Do not create cyclic card links on comment ref (https://github.com/nikku/wuffle/issues/143)
• Update iconography / colors to new GitHub style
• Stub keyboard shortcuts link (https://github.com/nikku/wuffle/issues/140)

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Closes https://github.com/nikku/wuffle/issues/143 Closes https://github.com/nikku/wuffle/issues/140