Layout of the pre-upgrade report page

The current thinking of the layout for this page comes from https://www.drupal.org/node/2281691#comment-10338513:

Tagged for Barcelona sprinting.

Looks a lot better than the original, thanks!

1. +++ b/src/Form/MigrateUpgradeForm.php
   @@ -567,6 +567,8 @@ class MigrateUpgradeForm extends FormBase implements ConfirmFormInterface {
   +      case 'preupgrade':
   +        return $this->buildPreUpgradeForm($form, $form_state);
   

   This is supposed to be a replacement for the confirm form, not a whole new page. With this patch, pretty much the same information is on two consecutive pages, just presented differently.

2. +++ b/src/Form/MigrateUpgradeForm.php
   @@ -869,6 +871,145 @@ class MigrateUpgradeForm extends FormBase implements ConfirmFormInterface {
   +    $desc = <<<EOT
   

   Should be a normal string on a (very) long line - I don't see it formally forbidden in the coding standards, but grepping shows that heredoc is not a pattern used in core modules except in a few tests. I suspect that it interferes with the translation tools.

3. +++ b/src/Form/MigrateUpgradeForm.php
   @@ -869,6 +871,145 @@ class MigrateUpgradeForm extends FormBase implements ConfirmFormInterface {
   +The following items will not be upgraded because there's no defined place ¶
   +for them to move to. See <a href="https://www.drupal.org/upgrade/migrate">
   +Upgrading from Drupal 6 or 7 to Drupal 8</a> for ways to fix this.
   

   Not fond of this wording. "no defined place for them to move to" isn't technically accurate ("no defined process for moving them" would be more precise). "ways to fix this" implies that there's actually something wrong. Our biggest challenge in messaging around missing migration paths is that in most cases, there isn't *supposed* to be a migration path - many modules have no data/configuration that requires migration. The only case where there's something for a site administrator to "fix" would be if there is a migration path but the destination module is not enabled. See #2569771: [meta] Highlighting of source modules with no upgrade paths.

   At one point, didn't we decide to not even show this information? That would be analogous to update.php, which does not tell you about source modules which lack update hooks on the destination side...

One other thing, the destination modules are not displayed anywhere, I think that's more useful that the detailed list of migrations for each source module.

@quietone: With some stuff I committed yesterday (adding rollback capability) this no longer applies, could you reroll?

Thanks.

@quietone: Thanks, I'll review the code and test locally shortly!

Still concerned that the "missing" upgrade paths (which are mostly not problems at all) are being overemphasized by putting them up front - @yoroy?

We could actually hard-code the core Drupal 6 and 7 modules we know intentionally have no upgrade paths, which will help some - there's not much we can do about contrib though, there's no way the tool can know that.

There's no way the tool can know whether a source module has migratable data or not. The challenges around trying to figure out what to tell users for each module are described at #2569771: [meta] Highlighting of source modules with no upgrade paths.

I'm going to commit the current version, which is a definite step forward (and which I've already taken screenshots of for my upcoming blog post on the upgrade process;) - I'll leave this issue open for further refinement.

Any further UX refinement should go under #2678638: [META] Usability refinements for Migrate UI.