This is a major update to the existing datagrid columns, largely

preparing them for the new Dashboard API resource, and fixing up a

number of issues in their implementation.
All columns now provide a stable JSON representation that can be used in

the API, returning structured data, API-compatible objects (which will

be turned into payloads or links), or raw values.
In some cases, this is done by overriding the new to_json(), but in

most we just now calculate the state for both the JSON and HTML

representations in the column-controlled get_raw_object_value(). This

keeps the data calculation in one place, and simplifies both output

functions (often using default logic).
HTML rendering has been fixed in all places to correctly use

format_html() or escape() for results. This will future-proof these

columns for when the datagrid code starts escaping non-SafeStrings.
Most of the __init__() methods are now gone, instead using the new

class-defined attributes. This is much easier to maintain and

specialize.
Types have been fixed and annotated all throughout, catching issues in

some of the logic.
Many bugs (such as extra spaces at the end of values, wrong data types

being returned where strings are expected) that weren't previously

caught have been fixed, with tests updated to strictly check return

types.

Unit tests pass.
Tested all columns in the dashboard in both the browser and with

a test script using the in-progress dashboard API.