Bugfix/main padding top mbl fix

[klepas]

Description

main now receives the same padding-top at mobile sizes that it received at tablet+ sizes (fix for content hugging the top of main/the page header).

I first spotted this affecting the mygov alpha, and again just in implementing the uikit again for another site.

Additional information

Before:

After:

Definition of Done

• [x] UX reviewed by Gary or someone the Design team
• [x] Code reviewed by one of the core developers
• [x] Acceptance Testing
  • [x] HTML5 validation (CircleCI)
• [x] Stakeholder/PO review
• [x] CHANGELOG updated

[klepas]

Oh, and I added some formatting improvs (blank lines following an @include).

[petronbot]

Looks good - mergin'