tweak/multiple_dice_rules_sel

When using the dice rule selector on edit game page, merge rules into existing rules.

From post

In very complex situations (some of these codeblocks include character sheet integration and all that as well) it could become tricky to merge.

At least add a warning if a merge is not viable.

You can select and deselect multiple sets of dice rules. It doesn't stop you customising them too.

If a set of repository rules are a subset of the game rules then it counts as selected.

It automatically removes duplicates.

Looking good. Will let people tinker and get the rules they're looking for tailored to their needs.