Send Message Flow a11y Audit and Fixes

[sheevdave]

Epic Goal
When we implement this epic, new users will:
Be able to use the send message functions of the product using screen readers and other assistive technologies

Epic Why
To ensure Notify is section 508 compliant, accessible, and user-friendly for individuals with disabilities or users of assistive technology

To Complete

• - Send Message A11Y Audit - Back button #2123
• - Send Message A11Y Audit - Unique IDs #2125
• - Send Message A11Y Audit - Redesign Send Message Flow to Use USWDS Step Indicator #2126
• - Send Message A11Y Audit - Cancel button for CSV upload not announced to screen reader #2138
• - Send Message A11Y Audit - Make Scheduling SMS more accessible #2139
• - Send Message A11Y Audit - Make the Delivery status area accessible #2143
• - Send Message A11Y Audit - Error messages not reading correctly in VoiceOver #2124
• - Send Messages A11Y Audit - Rename H1s #2135
• - Send Message A11Y Audit - Small changes #2133

[heyitsmebev]

After working on this ticket: Send Message A11Y Audit - Unique IDs, we've realized that we also need a client-side validation on forms, which is noted here: #2214