Attribution should not use innerHTML=

Nobody has claimed this yet.

## Motivation

While working on #7914 to add `rel="noopener"` to cross-domain links, I noticed that attribution control links are [directly injected using `innerHTML=`](https://github.com/mapbox/mapbox-gl-js/blob/6d529c74ac3524749bfb50a4e6f5c991f4d058c7/src/ui/control/attribution_control.js#L160). For me this raises a couple of questions:

1. How can I get `rel="noopener"` added to these links? As it stands, this would have to change on the data source side. Is HTML really the best way to communicate attribution?
2. Using `innerHTML=` is a potential security issue. While mapbox is obviously a trusted 3rd party, there are certain use cases where having this opportunity for remote execution would be very undesirable. This is especially important if using third party sources – until I read the source code just now, I thought I was simply consuming their data, and didn't realize that I was essentially giving them full access to the contents of my page.

## Design Alternatives

1. Parse the HTML on the client side, and reconstruct DOM links accordingly. It looks like [mapbox-gl-native already does this](https://github.com/mapbox/mapbox-gl-native/blob/d5659aa6647f1fc77159567bd22029a2dc9cd7a3/platform/darwin/src/MGLAttributionInfo.mm#L20) to make its attribution links. This seems a bit silly to do, but is probably the best way of fixing this while being backwards-compatible.
2. The most straightforward fix is to change the source attribution format to separately describe the titles and URLs of each source. However, this would require a change to the spec and break all sorts of existing implementations.

Attribution links are built in src/ui/control/attribution_control.js; start there and compare the parsing approach in the linked mapbox-gl-native MGLAttributionInfo.mm. Resolve whether the backwards-compatible alternative or a source-format change is intended; done means attribution remains functional without direct innerHTML injection and the link behavior is covered by tests.

Written by the indexing model from the issue text.