Better element handling in `syntactic_block_item`.

Now all elements are handled within local.common.
Error if element used in incorrect context.

Should be less suprising than just not processing them. Needs more tests.
Error if phrase element body doesn't parse.
Allow block markup inside nested blocks (e.g. tables)

Since this is often indented, does not treat indented blocks as code
blocks. This is inconsistent for lists, so maybe indented blocks in
lists should always be code, or never be code. Or maybe it should be a
bit smarter about nested indented blocks.
Allow block markup in phrase templates.