Adjust ContributionChart coloring

[MrOrz]

Adjust coloring again to make contributions obvious for both low contribution and high contribution users.

• For users with max contribution >= 20, it takes 5 contribution to advance to next color.
  • This is the same as before add github like contribution graph css and ceil for 1,2 report count #453 is merged
• For users with max contribution <= 8, it only takes 2 contribution to advance to next color.
  • So that new users in editor's meetup can see their color deepen faster
• For users with max contribution in between, the coloring step scales propotionally.

Screenshots

In the screenshots below, the maximum daily contribution is 29.

Before #453

• Takes 5 contribution to advance to next level
• However, coloring is rounded (Math.round(count / 5)) so that there is no color for count = 1 or 2.

Current production (#453 )

• days with contribution 1, 2 lights up (by Math.ceil in add github like contribution graph css and ceil for 1,2 report count #453)
• However, some cell's contribution color become lighter because maximum color is scaled to 29.

This PR

• As max contribution > 29, it is colored as if max contribution is 20.
  • Advance to next color every 5 contributions.
  • Any contribution >= 16 will be colored in deepest color.
• As a result, the color of many cells are deepened.

Users with fewer contribution or no contributions

[coveralls]

Coverage increased (+0.07%) to 75.363% when pulling d808d14 on adjust-contrib-chart into 3b5ad97 on master.