feat: implement pagination

.gitignore

assets/js/page_image_detail.js

assets/js/page_upload.js

assets/sass/_navigation.sass

assets/sass/_page_image_list.sass

assets/sass/_pagination.sass

assets/sass/style.sass

errors.go

page_image_detail.go

page_image_list.go

page_index.go

page_upload.go

templates/_base.html

templates/_footer.html

templates/_header.html

templates/_navigation.html

templates/error.html

templates/image_detail.html

templates/image_list.html

util.go