Make "fridays" configurable per repository

[Qqwy]

For most source code repositories, restricting merges on Friday is important and makes sense.
For repositories that don't actually contain source code for a running service, such as a 'documentation' repo, it makes less sense.

However, currently, the Friday option is enabled everywhere-or-nowhere.

This can be improved by making a small change to the configuration file format to add an extra boolean flag in the per-repository settings, and skipping the Friday check if that flag is set.