Uragnite mixin adjustments

[CriticalXI]

I affirm:

• [x] I have paid attention to this example and will edit again if need be to not break the formatting, or I will be ignored
• [x] I have read and understood the Contributing Guide
• [x] I've tested my code and the things my code has changed since the last commit in the PR, and will test after any later commits

What does this pull request do?

Adjustments to the Uragnite mixin:

• They will now always use venom shell when forced into their shell with damage.
• Uragnites are forced into their shell by doing a certain % of their HP of damage rather than being a random chance
• They now also can go into and out of their shell on a timer, even when roaming.

All adjustments are from my own time spent poking uragnites on retail.

Steps to test these changes

Watch a uragnite in game to see that they now go into their shell randomly even when out of combat.