Adds buildable and upgradeable PSUs (battery racks, not SMES).

--This PR touches code/defines/obj/weapon.dm, if any PR appears refactoring that cesspool of a file, do NOT merge this.

Adds two kinds of battery racks:
-One researchable in R&D with power 3 and engineering 2. This one is as safe as a SMES.
-Another able to be built with very common materials. It may overcharge and accidents can happen due to that.

Playtest required to refine the timings of those accidents.

Thread: http://baystation12.net/forums/viewtopic.php?f=5&t=9514#p197554
Typo, "<span class='warning'>Turn off the [src] before.</span>" before what?