Remove Season IDs

The season id field was used in the past to differentiate between different seasons, for example the info page for 2023 from the one from 2024. The issue with using season IDs is that it leads to "weird" URLs. What is easier to read in a chat message between unityleague.ch/info/2024 or unityleague.ch/info/2 ?

The IDs were introduced as there is not a 1:1 mapping between year and seasons, as we might want in the future to switch to a different naming scheme, for example if we switch to having 6 month seasons. In order to work around this, I added a new string field to season, named slug. This field is used to construct URLs for a given season, and is intended to be human readable, e.g. '2023', '2024', 'season-vi', etc.

The rest of the changes are various routing changes to make it available for the existing per-year views.