Support Unicode block codes

Background

Block codes (aka slugs, aka block_ids, aka url_names, aka component_codes or container_codes) have historically been restricted to ascii slug characters (A-Za-z0-9_-). But, we recently discovered that we have some partial support for non-ascii characters in block codes in V2 content libraries: https://github.com/openedx/openedx-platform/pull/38402#discussion_r3121151211. Specifically, non-ascii alphanumeric unicode characters, i.e. the character class accepted by re.match(r'\w', unicode=True) in Python: https://docs.python.org/3/library/re.html**

Currently, as far as we know, you can only get a block with a non-ascii code by doing one of the following:

• Creating a container (unit, subsection, section) with an initial title that includes non-ascii characters (in a content library)
• Editing a component or container's key with the library backup ZIP, and then restoring that backup.
• Creating a component or container with a non-ascii alphanumeric character (e.g., "θ") in the title and then copy-pasting that component into a content library. The original item won't have a non-ascii code, but the pasted item will.

In the long term we'd like to support such non-ascii codes, but in the short term we're concerned about downstream effects of those codes making into systems which were designed without unicode in mind, like:

• modulestore
• courseware student module
• analytics

Acceptance criteria

• Research the effect of non-ascii alphanumeric chars on those downstream systems
• Decide which systems we feel are safe for such characters
• Document this decision
• Loosen UX-level validation as to allow for them
• Do the same for: library codes, course codes, org codes. Or create ticket(s) for them.

Start by tracing how non-ASCII block codes reach modulestore, the courseware student module, analytics, and UX-level validation. Research the same behavior for library, course, and organization codes, then document which systems are safe and identify the validation changes or follow-up tickets needed.