Replace all * portTICK_RATE_MS and /portTICK_RATE_MS by MS2TICKS and TICKS2MS macros

This has been bothering me for a while, and https://github.com/TauLabs/TauLabs/pull/479#discussion_r5308521 pushed me over the top.

1) Fixed all incorrect ticks-->ms and ms-->ticks conversions
2) Replaced conversions by macro. Strict usage of this macro
will prevent errors of this type.

There were lots of errors of this type, but because we were always dividing or multiplying by 1 it didn't produce any bugs. By moving to the macro, we can prevent future confusion over this portTICK_RATE_MS define.