[Feature] Add secret storage in project  #14

It's just a killer feature that PHPCI hasn't (but a lot of CIs has).

How it works:

• You add secrets in project settings
• They had been injected in system environment variables before project builds
• You can take them from env

Use cases:

Add keys, passwords to deploy or something like these.

Why it's better than inject them by env plugin:

Because of security. It's not good idea to store secrets in repo(it can be public).

What's needed to change

• Database schema. Add table for secrets and add connections between project and secrets.
• Project view. Add fields for it.
• Form builder. Add multiple inputs that can be fetched as array.
• Build logic. Add fetching and injecting(exporting).

What do you think?

P.S. Maybe I'll try to implement this if I have free time.