improve accessibility of login modal

[Tlazypanda]

Fixes #7967

Improved accessibility of Login Modal by modifying orphaned form labels for username and password. Changed suspicious link text 'here' to 'reset it here' for password reset. And added aria-labels for social icons.

• 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 #7968 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #7968   +/-   ##
=======================================
  Coverage   82.04%   82.04%           
=======================================
  Files          97       97           
  Lines        5642     5643    +1     
=======================================
+ Hits         4629     4630    +1     
  Misses       1013     1013           

Impacted Files	Coverage Δ
app/controllers/stats_controller.rb	73.62% <100.00%> (+0.29%)	⬆️

[jywarren]

This is great! Merging!

[jywarren]

Can you please give it one last test on stable.publiclab.org? Just because signup and log in are critical systems. Thank you!!!! Great work!!!!