Summarize results in custom and contrib categories separately visually

Problem/Motivation

The intention of upgrade status is to provide a high level overview and help people make decisions on what to do. A high level summary similar to the status report summary was on the original designs but we did not get there to implement that yet. This is how it looks on the status report:

The current UI looks like this with some parsed projects:

While it is possible to get an overview of these results, most real sites have a lot more modules. Having a summary of the results would therefore be useful.

Proposed resolution

UpgradeStatusForm already has this commented out code:

    // @todo Make the display better and more visual.
    /* $summary = [];
    if ($counters['known-errors'] > 0) {
      $summary[] = $this->formatPlural($counters['known-errors'], '@count total error found', '@count total errors found');
    }
    if ($counters['no-known-error'] > 0) {
      $summary[] = $this->formatPlural($counters['no-known-error'], '@count project has no known errors', '@count projects have no known errors');
    }
    if ($counters['not-scanned'] > 0) {
      $summary[] = $this->formatPlural($counters['not-scanned'], '@count project remaining to scan', '@count projects remaining to scan');
    }
    $build['summary'] = [
      '#type' => 'markup',
      '#markup' => '<div class="report-counters">' . join(', ', $summary) . '.</div>',
      '#weight' => -10,
    ];*/

This is commented out because it did look very odd and barely readable, so it was not providing value. Making it visual would let us return to displaying them for both categories.

I think we actually need 4 categories:

1. Known errors found (this is actual number of errors count, not projects with errors IMHO)
2. Known warnings found (these are problems that are not yet fixable, but should be fixed before Drupal 9, also should count actual warnings, not projects)
3. Projects that had no warnings/errors (this is counting projects)
4. Projects not yet scanned (this is also counting projects)

I think making it clear in the label that 1/2 is counting actual issues and 3/4 are counting projects should be enough. Also we already have icons for these in the table (the black X not shown for unscanned module).

Remaining tasks

Create twig template and css, add it to the right places. It looked like status report CSS/templates are not reusable as-is.

User interface changes

Category summary numbers would be added for custom and contrib modules.

API changes

None.

Data model changes

None.

Release notes snippet

N/A