Tag Implication: on_back, on_stomach, on_side -> lying

Implicating on_back, on_stomach, on_side -> lying.

Reason: The three on_* tags are particular forms of lying.

forum #56907
At least on_stomach is currently not only used for lying images.

In that case, the wiki entries should be updated to reflect actual usage of the tags, although a quick glance at on_stomach -lying, on_side -lying, and on_back -lying suggests that the vast majority of such posts are in fact incorrectly missing the lying tag.

I see two possibilities:

• We could have the on_* tags solely refer the positioning of the upper body, and then in addition use the lying tag for when the whole body is roughly horizontally placed on a surface.
• We could stick to the wiki's current definitions, and adopt the implications I suggested. Then we would have a few posts now incorrectly tagged (probably less than 100), and as well lose the ability to make the distinction articulated in the first bullet.

The second course of action has the virtue of simplicity and fixing the by far larger number of incorrectly tagged posts. The first allows for finer tagging. What would people prefer?