Follow Web Content Accessibility Guidelines (WCAG) on the webapp

Reproducing feature suggestion from a visually-impaired user, I'm not entirely sure if it applies.

Translated from Portuguese:

> In the web app, it's already possible to use a screen reading software (used by visually-impaired people) using keyboard commands. However, it's necessary to implement the guidelines from WCAG to improve your user experience, promoting accessibility. To give examples, 1) when going through the editing fields with the Tab key and screen reading software on, the names of fields are not announced; 2) when you open note to edit, the screen reading software doesn't announce that new content was loaded on the page.

### What
Make the app fully compliant to https://www.w3.org/WAI/standards-guidelines/wcag/

### Why
Because it aligns with Automattic's core values and makes Simplenote accessible to a new set of users.

User-report: #2106972-zen

Start by auditing the web app's editing fields and note-loading flow against the WCAG guidelines linked in the issue, using keyboard navigation and screen-reading software. Done means the reported field-name and newly loaded-content announcements work and the broader WCAG compliance scope is defined and addressed.

Written by the indexing model from the issue text.