[Button] Simplify less code

[ColinFrick]

Description

Currently the button.less has over 4600 lines of code, most of the code is need for the different colors which differ only in the used variables.
Simplifies:

• color / primary / secondary
• positive / negative

This doesn't change the built button.css except for:

• Removed comment
• Fixes typo in tertiary teal button (.ui.tertiarteal.buttons .tertiary.button)
• Adds missing definition for secondary button margin left (primary was duplicated)
  .ui.buttons:not(.vertical) > .basic.primary.button:not(:first-child) {

Closes

#128

[prudho]

Really neat 👍