[SM-724] improve table header a11y; fix vertical layout shift

[willmartian]

Type of change

- [x] Bug fix
- [ ] New feature development
- [ ] Tech debt (refactoring, code cleanup, dependency upgrades, etc)
- [ ] Build/deploy pipeline (DevOps)
- [ ] Other

Objective

Fixes Axe a11y issue "Table header text should not be empty" by moving show/hide logic to the th instead of the button.

Also fixes the vertical layout shift by using css visibility instead of *ngIf.

Code changes

• bitwarden_license/bit-web/src/app/secrets-manager/shared/projects-list.component.html: Updates show/hide logic in table header

Screenshots

Existing behavior:

Screen.Recording.2023-04-28.at.11.51.48.AM.mov

New behavior:

Screen.Recording.2023-04-28.at.11.50.56.AM.mov

Before you submit

• Please add unit tests where it makes sense to do so (encouraged but not required)
• If this change requires a documentation update - notify the documentation team
• If this change has particular deployment requirements - notify the DevOps team
• Ensure that all UI additions follow WCAG AA requirements

[Thomas-Avery]

Looks good, thank you!