Blocks with colons in delta do not render

Problem/Motivation

When a block has a delta string containing a colon is referenced in a blockreference field, it does not render.

Steps to reproduce

Define/identify a block that has a colon in its delta string. Reference it in a blockreference field that normally displays on content. View that content. The block does not show.

Proposed resolution

Fix the code to limit delta explosions. That sounds cooler than it is.

Remaining tasks

Review & commit patch.

User interface changes

None

API changes

None, assuming nothing relies on the bug.

Data model changes

None