Match the green colors in the help system

[bsoyka]

constants.Colours.bright_green and constants.Colours.soft_green both exist, and they're very similar. (For reference, orange and soft_orange are nowhere near each other.)

bright_green is only used 3 times: twice in the help channel system and once in the modpings cog. soft_green is used 19 times across 12 files.

I propose that we make all the green color usages the same, keeping only one and possibly renaming it to green.

[bsoyka]

After some #dev-contrib discussion, it might be easier to simply adjust one or two of the soft greens to use bright green.

The original intentions behind this issue were to make these two messages, which are in the same place and both related to the help system, match:

Instead of combining the two colors altogether, I'll change the helpdm message to use bright_green.