Add buttons to click through schools

[lchristmann]

As a user interested in seeing the different Waldorf schools in Germany (especially see the photos), I'd like to be able to click a school and to swipe (=click) through them, as such:

• This is similar to a carousel, but ofc it isn't a carousel - it will just be buttons with links to the next school's page.
  • https://portal.waldorfconnect.de/school/22 will have buttons to go to https://.../school/21 and https://.../school/23
• The layout of the heading and the two buttons can be described with justify-content: space-between. (use bootstrap though!)

[linusgke]

Looking at the new system where groups and schools are the same, we can't just increment or decrement the id, because unrelated groups could be in between.

[lchristmann]

Indeed. We should get the next school with a SQL query

SELECT * FROM `groups` WHERE `is_school` = 1 AND `id` > currentId ORDER BY `id` LIMIT 1

and the previous school with

SELECT * FROM `groups` WHERE `is_school` = 1 AND `id` < currentId ORDER BY `id` DESC LIMIT 1