feat: add no-request uri fallback

[connorhaugh]

Description

I ran into the following issue:

I’m building out a management command to import V1 content Libraries into V2 content libraries. I’m using import_blocks_create_task which works 99% of the way for V1 libraries.
I’ve run into an interesting conundrum. I can get all the data from V1 libs without issue. I can even make a library block which contains that data.
The import, however fails, because upon creation, it returns the metadata object about the block it just created using get_library_block (which makes sense).
that call makes a call to blockstore, obviously, which makes a bunch of calls, one of which is to _build_absolute_uri which pulls the request from CRUM middleware and builds the URI from that.
The issue is, there is no request in the CRUM middleware because this is a management command, not a user request…
consequently, I get AttributeError: 'NoneType' object has no attribute 'build_absolute_uri'

THe way I fixed this, is by changing _build_absolute_uri so that it can fall back to settings.LMS_ROOT_URL to build the absolute URI if there is no current request.