Migrate sitemap.settings.rss_front to SitemapSyndicateBlock's configuration; test, deprecate SitemapSyndicateBlock

Problem/Motivation

The Sitemap module defines a block plugin with ID sitemap_syndicate (i.e.: \Drupal\sitemap\Plugin\Block\SitemapSyndicateBlock), which appears to exist only to display an RSS icon that links to an RSS feed.

Before commit b35b30f from 2017-03-22, the location of this RSS feed was customizable (i.e.: similar to the Feed URL config on the Frontpage sitemap plugin) — the location to link to was stored in the top-level configuration sitemap.settings.rss_front. Now this configuration setting doesn't have a control, nor a configuration schema.

While I doubt anyone actually uses the SitemapSyndicateBlock, deleting it would be a backwards-compatibility break / regression, so we should leave that to 3.0.x. But we should also fix it here.

Proposed resolution

1. Write a test for the SitemapSyndicateBlock.
2. Update the SitemapSyndicateBlock configuration to have an rss_front configuration (both in the schema and in the block plugin class). Update the test to check that it can be changed.
3. Write an update hook to see if sitemap.settings.rss_front exists, and if so, migrate it to the new configuration. If not, see if we can migrate it from the Frontpage sitemap plugin configuration. If not, fallback to the default.
4. Mark everything related to SitemapSyndicateBlock as deprecated for removal in 3.0.x

Remaining tasks

1. Write a MR
2. Review and feedback
3. RTBC and feedback
4. Commit
5. Release

User interface changes

Adds the ability to set the rss_front config for the SitemapSyndicateBlock

API changes

None (only deprecations).

Data model changes

Migrate sitemap.settings.rss_front config to block-specific config.