Consider dropping 'extend daily limits' from custom study

Custom's study's "extend" option was a quick-and-dirty hack, that works by reducing the count of cards studied that day in order to trick Anki into thinking more cards are still available. This leads to confusing behaviour when users extend their limits above the currently-available card limit, or when they put negative values in.

When @RumovZ introduced per-deck limits, I believe the thinking at the time was that they would eventually replace the existing custom study functionality. Perhaps it's about time we think about doing that?

The issue names no files, tests, or entry points. Start by tracing the custom study “extend” option and the per-deck limits it may replace, then determine the intended behavior for limits above availability and negative values. Done means the maintainers have agreed on the replacement or removal scope and its behavior is covered by appropriate tests.

Written by the indexing model from the issue text.