Remove magic strings

[bsmmoon]

Currently there are far too many magic strings, and the system is prone to silent failure from typo, change, etc.

One suggestion is to make constant package files. For example, a library that needs Form related constants will need to have use FormConstants and use getter like getDefaultHeader() to use a constant. This way, changing the value will not break anything, and some mistakes like typo will be caught by the compiler.

[spiliot]

Can you give an example of a magic string?