Create new index page for heap dump creation

[daniel-beck]

Two small improvements to /heapDump:

• Create a page explaining the heap dump download a bit and a known limitation.
• Require POST to download heapDump.hprof. Not addressing a security issue, just seems nicer.

This does not address the following remaining issues (which can be done later):

• Discoverability (e.g. linking from /systemInfo and corresponding agent pages, or directly integrating it there)
• Determine beforehand whether it would succeed, and only offer download option if it is.

Testing done

1. Navigated to /heapDump and clicked the button. Download started.
2. Navigated to /computer/a1/heapDump of a connected agent, and clicked the button. Download started.
3. Navigated to /heapDump/heapDump.hprof (from @RequirePOST)and clicked the button. Download started.
4. Navigated to /computer/a1/heapDump/heapDump.hprof (from @RequirePOST)and clicked the button. Download started.

Proposed changelog entries

• Create an index page for heap dump creation.

Proposed upgrade guidelines

N/A

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Desired reviewers

@mention

Before the changes are marked as ready-for-merge:

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[NotMyFault]

/label ready-for-merge

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This PR is now ready for merge. We will merge it after ~24 hours if there is no negative feedback.
Please see the merge process documentation for more information about the merge process.
Thanks!