Various improvements to secret template html file

[gamedevsam]

• Hide password hint when no hint is provided.
• Decrypt contents when users press enter key.
• Use password input type with checkbox to display text characters.

[mprimi]

@gamedevsam thank you very much for this contribution. Unfortunately at the moment I'm pretty squeezed and I doubt I'll have time to review and accept. Details here: #29

[gamedevsam]

No worries at all, everyone understands where you're coming from. If this project continues to get interest you might want to consider handing off the reigns to someone else who's interested and has the bandwidth to maintain it.

For anyone else interested in using Portable Secret using the improvements made in this PR you can use it on my website: https://sambatista.com/secret