Change capitalization throughout for consistency

Following 18f's guidelines:
- Sentence capitalization for titles and headings, ex. Structure of this tool
- Title caps for labels that are nouns, ex. Company Name
- Sentence capitalization for labels that are actions, ex. Add section

Updates to be made:
- Currently, text field labels are already consistent.
- Page headings are currently using title case but should be using sentence case
- Section headings are mostly using title case but should be using sentence case

**Decision: Sentence case**

The issue identifies page headings and section headings as the text to review; start by locating those headings in the application. Update headings to sentence case while leaving the already-consistent text field labels unchanged, then verify that page and section headings follow the stated capitalization rules.

Written by the indexing model from the issue text.