Improve visibility of shard allocation issues

In situations when shards cannot be allocated and a table/partition shows a yellow/red state, it is difficult for users to troubleshoot the reason with information shown in the Admin UI.

To give a simple example, create this table: `CREATE TABLE t (a INTEGER) WITH ("routing.allocation.require.name" = 'abc');`. The allocation requirement cannot be fulfilled, so the table health shows as red in the Admin UI. However, the user cannot find out through the Admin UI what the actual issue is:
* The table overview says "Health: red", but provides no further reasoning
* In the "Shards" tab, the table/shards don't show

Only when querying `sys.allocations` manually, and looking at the `decisions`/`explanation` columns, the root cause becomes visible.

**Suggested improvement**: Make information from `sys.allocations` more visible in the Admin UI, so users can troubleshoot allocation issues on their own, without having to know about additional `sys` tables.