Refactored Drafts plugin

[johnwargo]

While studying the Drafts plugin for my own work, I realized that the current plugin had a lot more code than needed which affected readability and should have a slight impact on performance. This PR:

• Replaces the environment variable with the isServing Boolean (which I think is faster in comparisons).
• Moves the check for the serve or watch Boolean earlier in the comparison logic so if the build process starts with serve or watch, the code never even checks draft status because draft status doesn't matter at that point. This probably isn't a big deal, but I think it makes it clearer what's happening. With this refactored code, the same number of comparisons happen (I think).
• Renames logged to isLogged making it clearer that it's a Boolean (for readability) and to align the variable name with isServing.
• Moves the isLogged = true to the !isLogged check block so the value change only happens once instead of every time the watched site rebuilds. In the original, logged= true executed with every build which was unnecessary.
• Reduces the code size from 50 lines to 34 (without removing any comments).