Wizard step for block migration

Problem/Motivation

Building on the work done in #2224297: Add a destination handler for custom blocks, I propose we add a step to the "Import from Drupal" wizard that sets up a Custom Block migration automatically.

Currently this does not import block titles; because that's not part of the migration added in #2224297: Add a destination handler for custom blocks. This was not discussed in that issue, but I suspect it to be be due to the fact that block titles are not stored with custom blocks (they're stored with block placements) and thus, block titles can actually vary per-theme (meaning it is entirely possible for a custom block to have a different title in each theme installed on the old site). Note that if we want to implement something to import block titles, it would be best done in a separate issue, to avoid the discussion getting too far off track.

Proposed resolution

Add a "Blocks" step to the wizard, allow the user to select whether to import blocks.

Remaining tasks

1. Write a patch
2. Review, feedback, RTBC
3. Commit

User interface changes

A new step to import blocks is added to the "Import from Drupal" wizard.

API changes

None.

Data model changes

None.