Extract and refactor dropdown item rendering

Basic Intent: Allow all branch names without accidentally creating
layout or backstage DOM. e.g. a branch named `separator` should never
create a separator `li` element.  

Ideally, there should never be a string that could cause this kind of 
conflict.  

Implementation: All of `GitLabDropdown.renderItem` is extracted to a 
standalone module.  

To render a divider or separator, consumers must now pass in an object 
like `{ "type": "divider" }` or `{ "type": "separator" }`   

Notable choices:  
- All of the functions have a cyclomatic complexity of 3 or less
    - See: https://en.wikipedia.org/wiki/Cyclomatic_complexity
    - Note the "Correlation to number of defects" section
    - While software complexity may not have a directly causal
      relationship with defects, less complex software is generally
      easier to reason about, and **may** reduce defects.
      I personally try to maintain complexity of no higher than 3.