This reverts commit cae5ca3d.

Make constance configs usable in other settings

See merge request AlekSIS/AlekSIS!118

Introduces a utility function lazy_config() that can lazily bind
a constance config key. As an example, it is used to use the primary colour
configured through constance as the app's theme colour.

Advances #139

Resolve "Use or create external library for template mailer"

Closes #152

See merge request AlekSIS/AlekSIS!117

Add ckeditor

See merge request AlekSIS/AlekSIS!116

Refactor and redesign turnable template part

See merge request AlekSIS/AlekSIS!115

Make app settings merging generic

See merge request AlekSIS/AlekSIS!114

Implements a merge_app_settings() function that merges a named setting
from all installed apps into the original setting in core.

- Add a links to quick launch
- Update url schemas for timetable to remove the need of get params
- Add i18n and reformat

Resolve "Re-introduce data migrations"

Closes #137

See merge request AlekSIS/AlekSIS!112