Sort terms drop-down lists by start date

Terms are easier to select when they are sorted consistently.

Test Plan:
* Create 3+ terms in a different order than their start dates
* Verify terms are sorted in chronological order by start date:
  * /account/### (right sidebar under Course Filtering)
  * /account/### (click Add a New Course; in the dialog)
  * /account/###/users (right sidebar under Choose Term)
* Terms should be sorted the same way as the drop-down list in:
  * Course Settings > Edit Course Details
Improve efficiency of sorting term drop-down lists
Merge pull request #358 from sfu/fix-term_sorting

Sort terms drop-down lists by start date