Fix several tiny CSS bugs

Closes #4050

- Full-width invite user modal button
- `<select>` can be focused in Firefox
- Increase size of background images for avatars
- Add `-moz-osx-font-smoothing: grayscale;` to icons
Correctly close PSM

closes #4077
- Swap CSS toggling for new PSM closing method
Merge pull request #4079 from halfdan/4077-close-psm

Correctly close PSM
Merge pull request #4053 from PaulAdamDavis/css-bug-fixes

Tiny CSS bug fixes