gddo-server: fix expanding examples containing reserved characters

[jackwilsdon]

The href attribute is percent-encoded by html/template, which ends up
breaking how Bootstrap links the heading to it's target. We can instead
use data-target to explicitly name the target, without it being
percent-encoded.

Fixes #570

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Patch Set 1:

Congratulations on opening your first change. Thank you for your contribution!

Next steps:
Within the next week or so, a maintainer will review your change and provide
feedback. See https://golang.org/doc/contribute.html#review for more info and
tips to get your patch through code review.

Most changes in the Go project go through a few rounds of revision. This can be
surprising to people new to the project. The careful, iterative review process
is our way of helping mentor contributors and ensuring that their contributions
have a lasting impact.

During May-July and Nov-Jan the Go project is in a code freeze, during which
little code gets reviewed or merged. If a reviewer responds with a comment like
R=go1.11, it means that this CL will be reviewed as part of the next development
cycle. See https://golang.org/s/release for more details.

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Uploaded patch set 2: Commit message was updated.

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Uploaded patch set 2: Commit message was updated.

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Uploaded patch set 3: Commit message was updated.

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