Allow other modules / themes to alter the block content [#3447741]

Problem/Motivation

In Wordpress you can alter the raw block content before it's rendered using the render_block filter. It would be nice to have this for Drupal as well with an alter hook.

Proposed resolution

Invoke an alter hook in the GutenbergFilter::renderBlock() method just before returning the $block_content. MR incoming!

Issue fork gutenberg-3447741

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3447741-alter-block-content changes, plain diff MR !127
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