Add help text to expense classes in settings area

Purpose: Help users understand how expense classes function

User story statement(s):

Scenarios:

1. Scenario:
   • Given i have navigated to expense classes in finance settings
   • When looking at the expense class list
   • Then I see help text at the top the reads as follows:

• Expense classes can be created and edited below. Once you have created expense classes, you can navigate to the Finance app to assign them to your budgets as desired. Once an expense class has been assigned to a budget, users can encumber or expend money against the expense class