create sponsors table
add Sponsor model, spec and factory
add sponsors routes
add sponsors link in admin nav
add SponsorLogoUploader
Sponsor.mount_uploader :logo, SponsorLogoUploader
add Admins::SponsorsController and spec
Merge branch 'feature/sponsors-management' into develop
create announcements table
add Announcement model, spec and factory
Announcement.mount_uploader :image, AnnouncementImageUploader
add announcements routes
add announcements link in admin nav
add Admins::AnnouncementsController and its spec
Merge branch 'feature/announcements-management' into develop
create articles table
add Article model, spec ans factory
Article.mount_uploader :image, ArticleImageUploader
add articles routes
add articles link in admin nav