Fixes #1704.

2. Replace <<EventNameLink>> with either App.UI.slaveDescriptionDialog or contextualIntro as relevant to each call site.
3. Get rid of <<EventNameDelink>> (and <<SetPregType>> which is obsolete and unused).
4. Document the changes for event writers.

Swap order of relationship/rivalry and relation.  It was built to be easy to change/add/remove elements, so most of this commit is just changing the examples.

1. Add a new function to introduce a slave based on their relationship to an already on-screen actor (i.e. "her husband John" or "your daughter and lover Alice").
2. Add a function to return the PC relationship term (lover/wife/husband) for slaves in a relationship with the PC, and use it in the slave summary.

1. Make areRelated and relativeTerm work regardless of familyTesting.  This fixes some bugs right away, since they are already used unchecked a few places, and it makes it a lot easier to write generic descriptions and event text without breaking things.
2. Extend relativeTerm to handle sole parent and grandparent/grandchild cases.
3. Make relativeTerm return null instead of "some unknown blood relation" if no relationship could be found between two slaves.  This was never OK to output (i.e. "$his some unknown blood relation" doesn't read right), so it ended up just being tested for everywhere (which can be done more easily with null).
4. Remove duplicate code based on familyTesting when the only difference was whether areRelated and relativeTerm or their legacy alternatives were used.  There is a lot of this.
5. Remove hasRelationP entirely (it is superseded in all use cases by areRelated).

This reverts merge request !5960

SugarCube repository moved from BitBucket to GitHub migrating from
Mercurial to Git. Hence the update for the guide.

Teach SugarCube to avoid cloning the state object when history size is
equal to 1

Added economy dev notes