Determine label color based on lowercase name

**What does this MR do?**
Determine the label color base on case-insensitive name

**Are there points in the code the reviewer needs to double check?**
Nothing special

**Why was this MR needed?** 
Because when you have a label "Bug" it would not get the red bug color, since it would not match. With this fix, the label name would be downcased when parsing for color detection.

**What are the relevant issue numbers / Feature requests?**
Fixes: https://github.com/gitlabhq/gitlabhq/issues/6759