Perform captions generation in UTF-16 encoding

The indexes returned in recording events from BBB refer to positions
within a UTF-16 encoded string. Rather than attempt to untangle this in
the server (which might have a performance cost), it's easier to switch
the caption processing code to operate in UTF-16 encoding as well to
make it work consistently.

The PyICU library provides a UnicodeString type which is a UTF-16 string
similar to Java and JavaScript, but which supports all the python
indexing methods. It's fairly straightforwards to swap it in in place of
the types used previously, and works natively as an input to the ICU
line break iterator too.

Fixes #10531