Add common RelatedLinksFields allowing empty text

This adds a RelatedLinkItemFields Abstract class defining common fields
for Relate Links. These common RelatedLink fields existed in the
following:

SchoolPageRelatedLink, ProgrammePageRelatedLink, EventIndexRelatedLink,
ReviewPageRelatedLink, StandardPageRelatedLink, StandardIndexRelatedLink,
HomePageRelatedLink, JobsIndexRelatedLink, AlumniIndexRelatedLink,
ResearchInnovationPageRelatedLink, GalleryPageRelatedLink

so all were refactored to inherit from RelatedLinkItemFields.

One thing here is that the link field in the previous was optional - I
left it also optional in the base class but can somebody explain why it
had to be optional ? I think that it should've been required.

The link_text field in RelatedLinkItemFields is optional. To get the title
for the link the get_link_text property has to be used. This will check to
see if the link_text field has a value - if not then the title of the
related page will be used.

The sidebar_link.html template has been modified to use the get_link_text.
added offical colors to show
Merge branch 'develop' into feature/showrca2014
added new logos for each school and updated themes to load them
ongoing themeing of show
Merge branch 'develop' into feature/showrca2014
ongoing styling of show pages
Merge branch 'develop' into feature/showrca2014
Merge branch 'student-profile-migration' into feature/showrca2014
two new migrations to improve show standard page and map pages
ongoing show work
styling gallery of images
Merge branch 'student-profile-migration' into feature/showrca2014
rand gallery now uses postcard image
Added 'Research & Knowledge Exchange' to AREA_CHOICES
Added migration to merge 'Research' and 'Knowledge Exchange' areas together
Removed 'Research' and 'Knowledge Exchange' from AREA_CHOICES
Fixed crash in migration 213
Update to latest Wagtail (which removes custom RCA styles from rich text editor) and reapply RCA styles with the new insert_editor_css hook
Merge branch 'develop'