Historically, Column subclasses had to override __init__ and set

attributes there. This was a bit messy and error-prone, as it was common

to mix up positional and keyword arguments when subclassing, and made

for harder customization for more complex class hierarchies.
Now, subclasses can set these arguments directly on the class as

attributes. The base Column class has defaults for all these fields,

which can be overridden by subclasses and further overridden when

calling __init__(). Built-in subclasses also provide their own fitting

this pattern.
To make this work, the arguments in __init__() are all set to UNSET

by default. If UNSET, class attributes (or more complex default

calculations) are used, but if any value is explicitly provided, it will

be used instead.
To help avoid the mess of positional and keyword arguments in these

classes, all arguments other than label are now keyword-only, with

positional arguments deprecated and scheduled for removal in Djblets 7.
This is now the recommended approach for column subclasses. It has the

benefit of storing less state per-instance, as well, as we only set the

values on the instance if they're explicitly provided during

construction.

Unit tests pass.
Tested updating existing columns to use this, and verified the new

settings were correct.