Allow maintenance mode to be set in config.js

[ErisDS]

This change makes it possible to test maintenance mode by setting the following in config.js:

maintenance: {
  enabled: true
}

However, I have a hunch that setting maintenance mode regardless of whether there is going to be a migration can cause the first request on boot to be a Maintenance Error.

I think we need to change this to only set maintenance mode if there is work to be done.

refs #6976, #7019, #7125

• Ensure maintenance mode flag is set back to what is in config.js rather than defaulted to false on boot
• Remove stack trace from 503 errors
• Add error message to 503 error
• Ensure error page is rendered for Ghost-Admin on reload with 503

[kirrg001]

all changes look good to me 🎉
one little thing: if the 503 page is rendered, there is an anchor go to the front page, which is in this case maybe a bit confusing.

[ErisDS]

Totally agree about the go to front page thing being confusing, I think that's actually true for a lot of cases and we're going to need to seriously improve how errors are rendered across the admin vs blog very soon.

For this release, I can't think of a quick fix that doesn't involve adding extra booleans or a helper to errors to be able to detect the error type / location, which then impacts on the theme API. Seems like overkill for this release but definitely want to revisit error rendering sooooooon.

[ErisDS]

P.S. I updated this to be one commit and have a full commit message