Avoid mutating string literals

[tommy-gilligan]

• Makes the tests a bit more accurate in replicating what output_buffer looks like in a Rails app.
• Adds # frozen_string_literal: true to all *.rb files
• Replaced string literal mutation in as many places as possible

[tommy-gilligan]

Needs #1340 for green build

[mikz]

I think this looks good.

I'm a tiny bit worried that this might affect some user code. Like custom inputs & actions. If someone inherits & extends those they would need to stop mutating strings too.

Have you had any issues with this in activeadmin?

[mikz]

@tomgilligan hey, I pushed to your PR.

Reverted the change of string concatenation instead of mutation. Rails methods like content_tag return mutable strings, so all code paths can rely on them. We should just keep doing the same thing in all code paths.
It could be a massive change for user code that actually relies on these mutable strings.