Add button to show and hide password in the devise registrations form

[linuxonrails]

Makes it easy for a user to check that he have typed the password correctly.

Checklist

• I have read the Contribution & Best practices Guide.
• My branch is up-to-date with the upstream master branch.
• The tests pass locally with my changes.
• I have added tests that prove my fix is effective or that my feature works(if appropriate).
• I have added necessary documentation (if appropriate).

Short description of what this resolves/which issues does this fix?:

• Makes it easy for a user to check that he have typed the password correctly.

Changes proposed in this pull request:

• Add button to show and hide password in the devise registrations form