Add searchfox links for metrics about glean-dictionary HOT 2 CLOSED

Let's try to implement this for the MVP, need to spec out a few things before it's ready for a contributor to tackle though.

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I think this is ready to look at. We can start just by searching searchfox (which has most of the products we care about, see "repositories" at https://searchfox.org/) for metric names.

I think mostly this is about creating a searchfox link in the metrics page. This is analagous to the view that the probe dictionary has:

https://probes.telemetry.mozilla.org/?view=detail&probeId=histogram%2FA11Y_CONSUMERS

We could probably use a more descriptive title than "Find in", but that can happen in a followup.

Notes:

• I think for an MVP of this we can create a manual mapping of products to searchfox URLs in this repository. Later we could investigate moving that metadata into probe-scraper
• For Firefox desktop, search indexing can work with this type of query: https://searchfox.org/mozilla-central/search?q=fog.ipc.replay_failures
• Many other products are indexed under https://searchfox.org/mozilla-mobile/source/ -- you'll want to restrict to the path when searching e.g. for Firefox Reality's distribution.channel_name metric you would want this: https://searchfox.org/mozilla-mobile/search?q=distribution.channel_name&path=FirefoxReality
• If a metric is expired and no longer in the source repository, the search will fail. #23 will make this a less obvious footgun (since it will be harder to discover obsolete metrics)

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