[openstreetmap/openstreetmap-website] Upgrade to Rails 6.1 (#3021)

[Andy Allan]

[Rails 6.1 was released earlier this month](https://weblog.rubyonrails.org/2020/12/9/Rails-6-1-0-release/), and will allow us to make two big changes:

* Use different buckets for different attachments. This means that we can store user images in one bucket (like now) and e.g. GPX files in another. Moving GPX files from filesystem storage (NFS) to object storage is a key item for the OSMF OWG too, to help with scaling and resilience.
* Use read-only database connections - see #2634 

Another feature in Rails 6.1 that I would like to explore is:

* Strict Loading Associations. Rails normally loads associations lazily, but that can easily lead to N+1 queries. I want to look at turning on Strict Loading, so that we need to be careful about declaring associations that should be preloaded, and reducing the number of queries that each rendering uses.

The current blockers for the upgrade is:

* Composite Primary Keys. There is a [pull request](https://github.com/composite-primary-keys/composite_primary_keys/pull/531) available, so if that gets merged we should be all set.
* I don't know of any other blockers yet!

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