improve accessibility of notifications settings

[Tlazypanda]

Fixes #7994

Improved accessibility of notifications settings by adding form labels for the empty form labels by adding hidden text.

• PR is descriptively titled 📑 and links the original issue above 🔗
• tests pass -- look for a green checkbox ✔️ a few minutes after opening your PR -- or run tests locally with rake test
• code is in uniquely-named feature branch and has no merge conflicts 📁
• screenshots/GIFs are attached 📎 in case of UI updation
• ask @publiclab/reviewers for help, in a comment below

Screenshots

Before

After

Thanks!

[codecov]

Codecov Report

Merging #7995 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #7995   +/-   ##
=======================================
  Coverage   82.04%   82.04%           
=======================================
  Files          97       97           
  Lines        5643     5643           
=======================================
  Hits         4630     4630           
  Misses       1013     1013           

[cesswairimu]

Looks good 🎉 Thanks @Tlazypanda