Set due date + FSRS + forced interval

[dae]

With SM2, a user can change the due date of a card, and optionally adjust its interval at the same time. e.g. change it from being due in 10 days to being due in 3, and optionally make subsequent intervals be based on an interval of 3 instead of 10.

With FSRS, the interval is ignored and stability is used instead, which means "3!" and "3" currently do the same thing as far as FSRS is concerned.

Originally reported on https://forums.ankiweb.net/t/set-due-date-hide-set-interval-to-same-value-if-fsrs-is-enabled/53890

[user1823]

"3!" and "3" currently do the same thing as far as FSRS is concerned.

True, but the reality is slightly more complicated when you also consider the effect on the calculation of R during sorting & searching. Reproducing some explanation from my comment in AnkiDroid repo:

Regarding the value of the checkbox:

FSRS user:

• No effect on scheduling
• Affects searching/sorting. But neither true nor false would result in correct behaviour. Correcting the sorting/searching requires either
  • changing value of ivl to the exact number of days between the due date and the last review date (43 days in my example above)
  • storing the value of LRD in the cards table, which dae has already agreed to.

Originally posted by @user1823 in ankidroid/Anki-Android#17707 (comment)

Though storing the value of last review date in the cards table would solve the issue, it will also increase the size of the database, which is already a concern for some people.

So, I tried to implement the other option in #3688, incorrectly believing that it should be trivial to implement. If someone knows how to get that PR to completion, please feel free to do so.