More freedom for styling the under construction page

[anselmdk]

I ran into 2 issues when styling the under construction page:

• I couldn't find a way for setting up a separate template
• All requirements would be loaded

Thus these changes. Don't know if you want them part of your repo? They help me.

You can clear the requirements with this setting:

UnderConstructionErrorPage:
  ClearRequirements: true

[anselmdk]

I also just added the feature to set allowed groups.

[frankmullenger]

@anselmdk thanks heaps for this, I'll try to find some time soon to merge etc.

[anselmdk]

Cool. Note that there are now 2 configuration options: clear_requirements (true/false) and allowed_groups (array).